Speedo vs GPS

[rocket rod]

I've just returned from a long trip and compared the trip meter/speedo on the car (MY16 D4) with a GPS running OziExplorer. Trip meter says 9198Km, GPS says 8696K. That's a difference of roughly 500kms! Which one is right? If it is the car that is wrong how can I change it? The only change I've made is using GOE wheels with 265/60 R18 but I believe the circumference if these is not much different to the stand 19" tyres.

[BobD]

I've compared mine against the distance pegs on the side of the road and it was spot on with standard tyres and now about 5% low with larger tyres. My speed was high with standard tyres and now spot on with the larger tyres.

[simmo1]

GPS derived speed is correct, virtually cannot have error if travelling at steady speed. It depends on the refresh rate when you are changing speed. GPS derived distances I believe will be correct as well.

Vehicle odometers (distance) are quite unreliable in my experience usually with a constant scale factor. Vehicle speedo's likewise. Most are configured to run at about 6% error, ie 100km indicated = 94km real. This error is not neccessarily proportional over the speed range though. Every time I have checked my GPS speedo against one of the overhead speed radars they are always showing the same as the radar. This has been with maybe 6 different GPS units and with many of the overhead radars.

So in short the GPS odometer will be nearly exactly correct, the vehicle odometer is another storey though.

Cheers, Simmo

[~Rich~]

I've compared mine against the distance pegs on the side of the road and it was spot on with standard tyres and now about 5% low with larger tyres. My speed was high with standard tyres and now spot on with the larger tyres.

Yep 285/60 R18's are pretty close to accurate, standard sizes like any vehicle are always under.

[LRD414]

I've compared mine against the distance pegs on the side of the road and it was spot on with standard tyres and now about 5% low with larger tyres. My speed was high with standard tyres and now spot on with the larger tyres.
I've had the same experience as Bob, except mine is a 1-2% margin because I have the 265/60/R18 which is only 1-2% different from the 255/55/R19

Scott

[cjc_td5]

GPS derived speed is correct, virtually cannot have error if travelling at steady speed. It depends on the refresh rate when you are changing speed. GPS derived distances I believe will be correct as well.

Vehicle odometers (distance) are quite unreliable in my experience usually with a constant scale factor. Vehicle speedo's likewise. Most are configured to run at about 6% error, ie 100km indicated = 94km real. This error is not neccessarily proportional over the speed range though. Every time I have checked my GPS speedo against one of the overhead speed radars they are always showing the same as the radar. This has been with maybe 6 different GPS units and with many of the overhead radars.

So in short the GPS odometer will be nearly exactly correct, the vehicle odometer is another storey though.

Cheers, Simmo

Just be careful that from memory the odometer and speedo do not use the same calibrations. For instance the odometer may run true but the speedo is set in the CCF to run with a % error.

[Graeme]

Just be careful that from memory the odometer and speedo do not use the same calibrations. For instance the odometer may run true but the speedo is set in the CCF to run with a % error.
Absolutely! If you check at what displayed speed the vehicle will drop out of off-road height you will probably find it's around 53 kph, being actual 50 kph but modified for display by 2 kph + 3.5% because it has calculated the proper speed using the correct distance.

My notebook's GPS displayed speed varies near trees, jumping up and down by several kph.

[DeanoH]

I've driven three Mazda Bravo/BT-50's over longish distances and all had a speedo inaccuracy of 8-10 %. :o

My current Bravo came with 185R14's and the speedo reads 109 Km/hr when the GPS tells me I'm doing 100 Km/Hr. 'Upgrading' the wheels and tyres to 205/75 15's has resulted in the speedo reading 100 Km/Hr when doing 99 Km/Hr which is good enough for me. :) The resultant change in gearing (manual transmission) makes the vehicle much more driver friendly as a day to day, non loaded, drive. The handling and comfort is also markedly improved.

My cynical view is that the manufacturers engineer this deliberately to give the impression that the vehicle(s) give much better fuel consumption than they really do. :(

Why do you think the hire car companys love BT-50's. I had a major blue with a hire car company in Mildura several years ago when I deducted 10% from the distance travelled. :D They pretended to be shocked and horrified but the bastards knew all along and caved when push came to shove. Not bad when they're creaming an extra 10% on the odometer reading.


Deano :)

[letherm]

I wonder what would happen with a warranty issue if it was a couple of hundred kms out of warranty ?

Martin

[Nicky]

I've just returned from a long trip and compared the trip meter/speedo on the car (MY16 D4) with a GPS running OziExplorer. Trip meter says 9198Km, GPS says 8696K. That's a difference of roughly 500kms! Which one is right? If it is the car that is wrong how can I change it? The only change I've made is using GOE wheels with 265/60 R18 but I believe the circumference if these is not much different to the stand 19" tyres.

GPS measures distance between points, speedo measures distance travelled over the road, path or track, up and down hills etc. GPS distance measure will always be less than speedo, depending on the vertical and horizontal deviations of the path.

[Hugh Jars]

^^ and will only give an accurate speed when travelling in a straight line on level ground:)

[theresanothersteve]

GPS measures distance between points, speedo measures distance travelled over the road, path or track, up and down hills etc. GPS distance measure will always be less than speedo, depending on the vertical and horizontal deviations of the path.



There's also a lag as satellites are acquired, distance travelled during that time isn't recorded.

Speaking to a couple of service advisors, they reckon speedo is out but the odometer is correct. I can believe that, my Commodores were always 2 kmh fast and the VW is 5%. Connecting an elm327 and the speed from software reading the diagnostic port is much closer to gps derived speed.

[simmo1]

GPS measures time and distance between points or whenever it refreshes. The total distance shown is the sum of all the distance intervals. It is not the distance between start and finish. As for slope distance versus corrected to be flat I believe that the gps derived distance is the slope distance. Basically they measure lat and long and height. I havent actually tested the distance as to slope v flat, something I should do if even just for ****s and giggles.
Simmo

[Narangga]

I've driven three Mazda Bravo/BT-50's over longish distances and all had a speedo inaccuracy of 8-10 %. :o

Deano :)

That's because around -8% is what the manufacturers build in so that the speedos do not over read and therefore contravene ADR 18/03.

[Meken]

^^ and will only give an accurate speed when travelling in a straight line on level ground:)


& not in tunnels [emoji13]

[DeanoH]

That's because around -8% is what the manufacturers build in so that the speedos do not over read and therefore contravene ADR 18/03.

I've heard this excuse before and quite frankly don't believe it. My 2004 Disco 2a's speedo was spot on. My 1966 SIIA speedo was more accurate than this. I don't believe that the auto manafacturers are incapable of designing/mass producing a reasonably accurate speedo. To blame gross inaccuracy on ADR compliance is IMO a Furphy.

Deano :)

[Fluids]

My 2008/2009 D-Max came std with 245/70.16 ... Compared against the GPS in a straight line run, the speedo says 100kmhr but the GPS says 93/94kmhr ... I changed the tyres to 265/70.16 and now the GPS says 99/100kmhr when the speedo says 100kmhr ....

... Then I notice sometime later that most of the rest of the world markets get the D-Max with 265/70.16 std ... But in Oz it's the 245's ... I believe ADR's force manufacturers to have speedo's that over-read

Mum and Dads MUX is out by around 6-7% ... Wife's Subaru Impreza at around 118/119 on the speedo shows 109 on the GPS... My 2004 D2a at 100 on the speedo shows 106 on the GPS and 106 on the Nanocom with the tyre coefficient set at 1.06 ... Tyres ARE 6% oversize. Older cars seem to be accurate but newer vehicles ALL read more than actual .... Dad drove the D-Max for 5 years, and when he handed it over to me, the trip computer (that had never been reset in that 5yrs) said 7.4L/100Km ... With the 265's fitted I can't get anywhere near 7.4 ... after I figured out the speedo error issue I now understand why he was getting 7.4 :p

I put more faith in the GPS than the speedo ... Apparently we're not fit to drive a vehicle at 100 ;)

[DiscoMick]

The speedo must never read less than the actual speed by law. So, if the speedo says 100 km/h, you're probably doing about 96.
The odometer should be pretty accurate, but a GPS should always be spot on to within maybe 0.5 metres.

https://www.racq.com.au/cars-and-driving/safety-on-the-road/driving-safely/speedo-accuracy

Sent from my GT-P5210 using AULRO mobile app

[Bytemrk]

Mine (running standard size tyres and rims) is out by 6Km at 100.

If i sit the speedo on exactly 106 ...by GPS I am doing 100 ;)

[Grumbles]

In the driveway there are 2 x 3 year old cars and one x 22 year old. All read less than than the speedo. One of the 3 yo's has 2 x GPS fitted - one factory and one aftermarket - and these always agree with each other except for a minor difference of 1 kmh.

[Graeme]

Mine (running standard size tyres and rims) is out by 6Km at 100.

If i sit the speedo on exactly 106 ...by GPS I am doing 100 ;)
Fits the Oz CCF definition of 2 kph + 3.5% pretty well. There is an alternate option of 2%.

[AK83]

I recently did a trip in my D1.
Previous owner sold it with brand new 255/70 16's.
My Odo is +8% out according to OziExplorer too.
So for every 108klms that the car shows, I'm traveling 100klm road distance.
I verified this over a recent 2000+klm trip last month.
I've never had a vehicle so vastly different in Odo vs speedo calibration.
I think max was about 3%. Even my 79 RR was close to spot on(maybe about 1% out, but close enough).

The device you run OziExplorer on can also make a difference to how accurate the trip meters are.
I think you can configure up to 3 different trip meters too if you want.
(that's my current setup).
The small spikes in speedo readout make no difference to the trip meter's accuracy tho.
There are a few things that can make your (Ozi)GPS device's trip meters inaccurate.
If you've changed the default refresh rate. Default is 1Hz.
Some folks don't need or want such detailed tracklogs.
At 100km/h that refresh rate equates to about an update(and tracklog point) every 50m.
You can slow down the refresh rate, and on a faster road the accuracy is still pretty much spot on, but on a twisty windy slow road it can suffer.
The other thing that can mess up your GPSs trip meter reading is 'float'.
This is if you have the GPS stationary for any length of time.
eg. say you drive 50klms, and you find a nice spot for a bushwalk.
You park the car with GPS in it, and the GPS is left on.
If you're gone for a few hours on this bush walk, the GPS can drift wildly.
It drifts more the worse the satellite reception is(eg. under trees, ina canyon, etc).
Even out in the open air(eg. Lake Eyre).
If your trips are short and the GPS on static times are long, then the drift can add up to a significant amount.

What device are you running OziExplorer on?
Is the device using a built in GPS, or a tethered one?

[Narangga]

I've heard this excuse before and quite frankly don't believe it. My 2004 Disco 2a's speedo was spot on. My 1966 SIIA speedo was more accurate than this. I don't believe that the auto manafacturers are incapable of designing/mass producing a reasonably accurate speedo. To blame gross inaccuracy on ADR compliance is IMO a Furphy.

Deano :)

2004 and 1966 were before the 2006 update to ADR 18/03.

Hire any car from one of the major firms (which will be running standard rubber) and check the speedo and I bet it will show 100 on the speedo when it is actually doing 92.

Call it a furphy or conspiracy but it is the actuality - mainly because of the litigious world we now live in unfortunately.

[DiscoMick]

This all has implications for radar and speed limits, of course.
For example, I understand in Qld (not sure about other states) the radar is set not to trigger until the actual speed exceeds the speed limit by more than 3km/h. So, if your actual speed is 104 km/h in a 100 km/h zone the radar goes off.
But, if your speedo is reading higher than the actual speed that is out.
For example, at an actual 104 km/h your speed might be reading say 108 km/h. At an actual 103 km/h your speedo might read say 106 km/h.
So, in theory you could drive at a speedo reading of 106 km/h in a 100 km/h zone and not trigger the radar.
People sitting on 100 km/h by their speedo might be travelling at 6-7 km/h below the speed at which the radar triggers, after allowing for both the speedo error and the radar buffer zone.
Don't try this of course, but it's interesting to know.

[rocket rod]

I recently did a trip in my D1.
Previous owner sold it with brand new 255/70 16's.
My Odo is +8% out according to OziExplorer too.
So for every 108klms that the car shows, I'm traveling 100klm road distance.
I verified this over a recent 2000+klm trip last month.
I've never had a vehicle so vastly different in Odo vs speedo calibration.
I think max was about 3%. Even my 79 RR was close to spot on(maybe about 1% out, but close enough).

The device you run OziExplorer on can also make a difference to how accurate the trip meters are.
I think you can configure up to 3 different trip meters too if you want.
(that's my current setup).
The small spikes in speedo readout make no difference to the trip meter's accuracy tho.
There are a few things that can make your (Ozi)GPS device's trip meters inaccurate.
If you've changed the default refresh rate. Default is 1Hz.
Some folks don't need or want such detailed tracklogs.
At 100km/h that refresh rate equates to about an update(and tracklog point) every 50m.
You can slow down the refresh rate, and on a faster road the accuracy is still pretty much spot on, but on a twisty windy slow road it can suffer.
The other thing that can mess up your GPSs trip meter reading is 'float'.
This is if you have the GPS stationary for any length of time.
eg. say you drive 50klms, and you find a nice spot for a bushwalk.
You park the car with GPS in it, and the GPS is left on.
If you're gone for a few hours on this bush walk, the GPS can drift wildly.
It drifts more the worse the satellite reception is(eg. under trees, ina canyon, etc).
Even out in the open air(eg. Lake Eyre).
If your trips are short and the GPS on static times are long, then the drift can add up to a significant amount.

What device are you running OziExplorer on?
Is the device using a built in GPS, or a tethered one?

I use a Samsung Galaxy Tab about 4 years old with built in GPS and the settings are all default so I think that the unit will be recording accurately. I get what you are saying about drift while stationary as I saw the trip meter adding distance while standing still. I usually stopped the trip meter (turned the unit off overnight) so I think the cumulative error would be relatively small compared to the travelled distance.

Next is how to correct the error? What are the values in the CCF I need to edit to give me the correct odometer reading? Also the fuel consumption is out by quite a margin, is there a method to correct that as well?

I have been in contact with GAP to see if I can change the speedo reading and that say that with the IIDTool it is not possible to do that.

[BobD]

As per several posts, the odo in a D4 is accurate. It is the speed that is out. Check it against mileage pegs on the road before you muck around with it, not a GPS. Mine reads 100km for 100km of km posts, so dead on, but the speed is out by 6kph at 100, until I fitted bigger tyres and mucked up the odo.

[BMKal]

Mine (running standard size tyres and rims) is out by 6Km at 100.

If i sit the speedo on exactly 106 ...by GPS I am doing 100 ;)

Mine was exactly the same when I still had the OEM supplied Continentals on Mark. ;)

Since fitting the Mickey Thompsons, which are slightly larger in diameter, I'm now out by 3km/hr at around 100kph. If I set the cruise control on 114, the GPS (Hema HN7) sits very consistently on 110 - the greatest variation I ever see when going over hills etc out on the open road is 1 km/hr.

[BMKal]

That's because around -8% is what the manufacturers build in so that the speedos do not over read and therefore contravene ADR 18/03.


I've heard this excuse before and quite frankly don't believe it. My 2004 Disco 2a's speedo was spot on. My 1966 SIIA speedo was more accurate than this. I don't believe that the auto manafacturers are incapable of designing/mass producing a reasonably accurate speedo. To blame gross inaccuracy on ADR compliance is IMO a Furphy.

Deano :)

Dale is spot on. ;)

All new cars now read greater than the true speed in order to maintain compliance with ADR 18. Prior to July 2006, an error of +/- 10% was acceptable. Since that date, the allowable error has been increased to 10% plus 4 km/hr, but the speedo must always read "safe". In other words, the error can only be such that the speedo indicates a reading equal to or greater than the actual speed travelled.

https://www.aulro.com/afvb/images/imported/2016/08/770.jpg (https://postimg.org/image/ukip9klwl/)

[LRD414]

..... Since fitting the Mickey Thompsons, which are slightly larger in diameter .....
My Dad would love to go back to Mickey T's. What size/spec are you running Brian?

Cheers,
Scott

[Bytemrk]

Mine was exactly the same when I still had the OEM supplied Continentals on Mark. ;)

Since fitting the Mickey Thompsons, which are slightly larger in diameter, I'm now out by 3km/hr at around 100kph. If I set the cruise control on 114, the GPS (Hema HN7) sits very consistently on 110 - the greatest variation I ever see when going over hills etc out on the open road is 1 km/hr.

Mine has been exactly the same with both the Hankook AT/m I had and the current Goodyear Duratracs

[Nicky]

[QUOTE=theresanothersteve;2571374]There's also a lag as satellites are acquired, distance travelled during that time isn't recorded.

The lag is between bursts of data collection which reference the way points, NOT between way points.

[AK83]

I use a Samsung Galaxy Tab about 4 years old with built in GPS ....

I'm currently using a Galaxy Tab2 (7").
Good unit. Only reason I've kept with it was that it worked so well.
No reason to dump something that works!
(I'm a GPS geek/freak/habitual updater!)

I'm 99.999% confident that on an open road(no matter the weather either) .. you could safely put money on the GPS and Ozi being more true than the Landrover's odometer.

Sorry can't help with info about how to readjust settings in the car.

ps. have you looked at or played with the Ozi Screen designer?
If you don't know of it, or what it does:
Small peripheral program you load onto a PC(Windows!).
You're configure the screens you want on the device running Ozi.
I've changed all the default screens, added more info to most, added a few more for specific purposes.
One of the ones I use regularly when on a trip is one screen specifically set up with odometers.
I've set three of them, one I use for the total trip distance, another I use for fuel fills(estimate to empty), and third one for keeping distance tabs on small bits of tracks here and there.
First two are self explanatory, but the third is simple.
Get onto some obscure track somewhere, only with the knowledge that at 12.5klms there is a turn off.
Instead of relying on depleted grey matter to remember a klm reading I know I'll forget, I use this third trip meter.

On my recent trip, I used that second trip meter to record all fuel distances.
5 fills for the trip, all Ozi trip meter readings were exactly 8% more than the D1's odo.

ps. the speedo is pretty much spot on, only showing 1km/h faster than actual.
Actual = both Ozi on the Galaxy Tab and the overhead gantry's in some selected locations on Vic freeways.

[BMKal]

My Dad would love to go back to Mickey T's. What size/spec are you running Brian?

Cheers,
Scott

STZ 265/60R18 Scott. Very happy with them. :D

[rocket rod]

I'm currently using a Galaxy Tab2 (7").
Good unit. Only reason I've kept with it was that it worked so well.
No reason to dump something that works!
(I'm a GPS geek/freak/habitual updater!)

I'm 99.999% confident that on an open road(no matter the weather either) .. you could safely put money on the GPS and Ozi being more true than the Landrover's odometer.

Sorry can't help with info about how to readjust settings in the car.

ps. have you looked at or played with the Ozi Screen designer?
If you don't know of it, or what it does:
Small peripheral program you load onto a PC(Windows!).
You're configure the screens you want on the device running Ozi.
I've changed all the default screens, added more info to most, added a few more for specific purposes.
One of the ones I use regularly when on a trip is one screen specifically set up with odometers.
I've set three of them, one I use for the total trip distance, another I use for fuel fills(estimate to empty), and third one for keeping distance tabs on small bits of tracks here and there.
First two are self explanatory, but the third is simple.
Get onto some obscure track somewhere, only with the knowledge that at 12.5klms there is a turn off.
Instead of relying on depleted grey matter to remember a klm reading I know I'll forget, I use this third trip meter.

On my recent trip, I used that second trip meter to record all fuel distances.
5 fills for the trip, all Ozi trip meter readings were exactly 8% more than the D1's odo.

ps. the speedo is pretty much spot on, only showing 1km/h faster than actual.
Actual = both Ozi on the Galaxy Tab and the overhead gantry's in some selected locations on Vic freeways.

AK83

Interested in your screen designs for Ozi as I use the Screen Designer app myself so PM me if you would like to swap ideas.

As to your summary, I agree that the car's odo is probably at fault all I need now is some way of correcting it? As someone said here before "I wonder what would happen if there is a warranty claim" given that the car reaches the 100,000km before it's actually gone that far!

[BobD]

Did you guys even read the posts from Graeme, me and others that the D4 odo is accurate and the speedo is out deliberately by a set amount? Or the posts re the inaccuracies of GPS distance measuring as you are doing?


I can only repeat, do a proper check of distance, independent of a GPS, to determine whether the odo is accurate or not. You can't just say it is inaccurate when you haven't properly measured the accuracy against a known distance.

[rocket rod]

Bob

I have read the comments by others here but not one of them "prove" they are correct. Can you show me documentation that says the distance markers on the roads are accurate? I used to be a surveyor and I have a reasonable understanding of these things. For instance what happens when Main Roads realign a road and cut out corners thus making the total road length shorter. Do they re-position every sign on that road so their distance markers are correct? Nope. They are guide only. Everyone on this forum says the speedo is inaccurate but everyone also says by varying amounts. So who is correct and how can they prove it? VW had very good emission standards until recently remember!

To try and clarify the situation I've asked Main Roads WA and the creator of Oziexplorer for some comments, so I'll wait and see.

[BobD]

I'm glad you read them at least Rod. I thought you must have been selectively reading posts to suit your point of view.


I have checked mine against distance posts several times, in WA and elsewhere in Australia, just because I can. I've checked individual post distances and also over long distances. Every time the distance is spot on. Are all mile posts around Australia going to be out by the same amount as the D4 odo? Obviously if there was a sudden difference between the odo and pegs the distances may have changed on that piece of road but I am talking about most roads where things haven't changed much in a fair while.


Graeme has looked at the settings of the D4. The odo has been set at the factory to be accurate and the speedo has been set to read higher than the actual speed in accordance with the ADR's by a specific amount. This difference is what most people experience, indicating that the D4 speedo is also accurate within the parameters set by LR. How can you therefore categorically say in your post that the D4 odo is highly inaccurate?


You even have larger than standard tyres which means the odo is actually measuring lower distances than what the car is travelling, not higher.


I will be very interested in your findings from MRWA and Ozi Explorer and the results of your own more meaningful checks on the odo using your surveying skills!

[AK83]

Did you guys even read the posts from Graeme, me and others that the D4 odo is accurate and the speedo is out deliberately by a set amount? .....

So what you are saying here is that every single one of the hundreds of thousands of D4's ever made, the odo is dead accurate, and that's that! :confused:

But the advice to use the roadside distance markers as a guide is perfectly valid.
That's what I do.
They may not be 100% accurate if viewed upon as a surveyor, but as long as they're accurate enough, or can validate one set of readings, then that should be good enough.
So the point isn't if the 50k sign to The Black Stump is actually exactly 50klms from the place called the Black Stump, as long as they are confirming one reading or the other, that's what's important.
My recent trip was from Melb to Maitland via Sydney 100% all freeway. Then back via Bathurst, and many back roads, before hitting the freeway again at Yass.
Ozi and the distance markers all said the same thing, car was out.
I know to use Ozi as my guide to "the next town" .. and importantly if I have enough fuel to make it.

I think that many of us have the intellect to understand that the odo and the speedo readings can differ on any vehicle, that's almost a given situation. But using the law of averages, I don't think it's improbable that at least one D4 somewhere in the world(maybe even in WA!!) has an odo that may be slightly out of whack.

@ rocket rod.
I'll shoot you a PM as soon as I can get a few moments spare.
I just bought the Hema + additional maps collection from Ozi(worth the $, if you can justify it!), and I'm trying to configure which maps I want on the PDA(64Gig card, but still trying to keep it slimmed down).
+ installing a snorkel, + doing some errands for siblings, + trying to get some R&R before I'm back at work .. + + kids!
:p

[BobD]

Some valid points AK83. However comparing a D1 odo to a D4 odo is a bit misleading. When I read your first post I was assuming you had a D4, not a D1.


Note that in your last post you said your D1 was 8% under the GPS and mile post distances but the OP's D4 odo was reading well over the GPS distance. In your first post on the subject you said the speedo was reading 8% above the GPS. Which one is it?

[Graeme]

So what you are saying here is that every single one of the hundreds of thousands of D4's ever made, the odo is dead accurate, and that's that!Every D4 will be the same unless different size tyres are used or the CCF has been altered because it's totally electronic and calculated from wheel speed sensor pulses.

Edit: Even mechanically-driven odometers of older vehicles all produce identical odometer readings for the same model/diff ratio/tyre size although variations can exist in their speedometers due to relying on magnetic forces across an adjustable air gap and a return spring that can have slightly different characteristics.

[AK83]

Some valid points AK83. However comparing a D1 odo to a D4 odo is a bit misleading. When I read your first post I was assuming you had a D4, not a D1.


Note that in your last post you said your D1 was 8% under the GPS and mile post distances but the OP's D4 odo was reading well over the GPS distance. In your first post on the subject you said the speedo was reading 8% above the GPS. Which one is it?

Sorry if I was misleading in any way.
Car odo is definitely under. It has to be the oversized tyres at work(plus some built in error into the odo)
The 255/70 tyres have obviously cancelled out the speedo error, and I'm thinking must have exacerbated any odo error.

Brain fade on my part: too much work, not enough R&R, and too many chores to have to deal with .. all conspired against me.

I have a spreadsheet for my car expenses, obviously including fuel use.
I record the actual odo reading in the appropriate cell, but also have another cell with a calculation for +8% for the klm reading, to keep a track of it.
As the tyres wear a bit, or I replace the tyres at some point in the next few years, I'll have recorded data on what it's doing.

There was no intent to mislead that my vehicle is a D1.



Every D4 will be the same unless different size tyres are used or the CCF has been altered because it's totally electronic and calculated from wheel speed sensor pulses....

Something I learned a long time ago is to not assume that just because it electronic or digital(ie. 0 or 1) that it's accurate!
I'm not a electronics tech head in any way(just an enthusiast type!) .. but I've gone through my fair share of hair pulling episodes as to why some electronic device won't cooperate with the theory.

Anyhow, interesting discussion and it reminded me to do some updates on my GPS :D

[Graeme]

Something I learned a long time ago is to not assume that just because it electronic or digital(ie. 0 or 1) that it's accurate!I was commenting on the possibility of differences between vehicle odometers, not the accuracy of the calculation, although I consider the accuracy very good judging by the freeway 5km odometer accuracy test markers.

[AK83]

..... I consider the accuracy very good judging by the freeway 5km odometer accuracy test markers.

I can't think of any here in Vic(but there may be a set out on the freeway to the east of Melbourne?)

I used the set heading to Sydney past Goulburn on my trip up to Maitland to confirm GPS or Odo, and like you said, they're spot on(with the GPS).

So on the basis that they're accurate, which lined up perfectly with the GPS(to within a few meters of inaccuracy traveling at 80k/h), I then used many of the 5klm roadside markers to each town/locality to continue confirmation of the GPS.
Again, over a couple of thousand klms, and at least 20-30 different sets of those 5klm distance markers both in Vic, but mainly in NSW, the GPS metering was confirmed.
My conclusion is that those 5klm distance markers are accurate enough to gauge the car's odometer accuracy too.
Of course the longer the distance being measured, means lower error rates.
The longest of these in a continuous series that I know of is the set heading back from Albury to Melbourne on the freeway.
(from memory) I think that this set is almost continuous for about 270klms(that's the last one I know of coming into Cragieburn).

The only point I'd like to make to counter your comment Graeme, is that you said:
" Every D4 will be the same ..... "

And why I made my comment that my experience has shown me that you're better off being suspicious of electronic equipment.

The theory is that every D4 should be the same ....
Unless they are all confirmed in some way, I wouldn't assume it to be true.

[rocket rod]

Sorry to resurrect this old chestnut but I did say I would enquire the WA Main Road Dept and the author of OziExplorer for their comments on the roadside markers and the accuracy of the GPS readings. I have just received back the following from MRD WA and thought I should post it. I have not received a reply from OziE yet and somehow I don't think I will.

The distances shown on the signs that display how far it is to Towns and Cities on the Main Roads WA road network are a guide only. As you rightly suspect, variations in distance are liable to occur from time to time due to road deviations, relocations and construction works. Main Roads does perform periodic reviews of signage and in some cases will make changes to the signs or their position.

The smaller 5km/10km increment signs that countdown the distance to Towns and Cities are known as Focal Point Markers. Whilst every effort is made to make sure they are placed as accurately as possible, they too provide only a guide to the distances involved.

Attached is a link to the Main Roads Distance book, which contains more information regarding Focal Point Markers and distance measurement in general.

https://www.mainroads.wa.gov.au/Documents/Distance%20Book%20Edition%2013%20-%20Final.RCN-D13%5E23745120.PDF

[BobD]

Rod,


You have misinterpreted what I said. I've never mentioned those signs and never used them to check the odometer. I would never treat them as an accurate odometer check. I am talking about the distance pegs all along major roads which used to be every mile and are now every 2 or 5 km from memory. These are the design chainages of the road and are accurate on the design centre line of the road.


With these pegs you can check over long distances. I have checked the D4 on standard tyres over a distance of 100 km a few times on these pegs and it was dead accurate. Not so accurate on my larger tyres, however!

[Graeme]

My conclusion is that those 5klm distance markers are accurate enough to gauge the car's odometer accuracy tooThe markers are placed specifically for odometer checking, as indicated by the signs stating such.

[Not_An_Abba_Fan]

To add a bit more conspiracy theory into the mix. I sold cars for a number of years. The information sticker on the windscreen that states fuel economy and emissions? Manufacturers build in speedo errors to attain stated economy figures. If the car is doing ~95km/h and the speedo says 100, you are more likely to achieve the stated fuel economy on the sticker. The higher the revs, the more fuel is used, so the speedo shows a faster speed than you are actually travelling to keep the revs lower, thus using less fuel.

[Tombie]

The sticker is a lab test, real world figures are not obtainable.

It's nothing more than a guide.

Like appliance testing - no way our Tumble Drier only uses 155kw per year. [emoji41]

[cjc_td5]

Rod,


You have misinterpreted what I said. I've never mentioned those signs and never used them to check the odometer. I would never treat them as an accurate odometer check. I am talking about the distance pegs all along major roads which used to be every mile and are now every 2 or 5 km from memory. These are the design chainages of the road and are accurate on the design centre line of the road.


With these pegs you can check over long distances. I have checked the D4 on standard tyres over a distance of 100 km a few times on these pegs and it was dead accurate. Not so accurate on my larger tyres, however!


The markers are placed specifically for odometer checking, as indicated by the signs stating such.

Graeme, in WA the main roads asset system uses posts along the road to indicate the SLK (Straight Line Kilometres) from the start of the road. They are like guideposts and spaced at 2kms, or 5km in the northern areas. You can follow the Gt Northern Hwy from SLK0km at Midland (Perth) to a post with 3600km on it near Wyndham. They are generally accurate enough for an Odo check. Other states use a Segment based system that is nowhere near as obvious to the layman using the road.

Bob, just be aware that the SLK chainages are not necessarily continuous. When a road realignment is done (say the road is made longer), the posts are NOT adjusted for the entire road length after that spot. There is a step made in the SLK system to account for the change in road length. You will see this in an anomaly between measured and post distances.

Anyway, that is me getting way too technical....:D:D:D:D
Cheers,
Chris

[BobD]

Chris,


I'm pretty certain I've seen the same chainage pegs in the NT as well but not sure of the other states. Sometimes you forget since WA is so big compared with the other states and I forget where I have seen them.


Thanks for the explanation. That is what I thought was the case, but I haven't talked to my colleagues in the road design department to find out exactly how they work. I am a structural engineer so we don't usually deal in such large dimensions!

[Graeme]

I should have included more of the text to show that I was responding to comments regarding the 5km odometer test strips commonly seen in NSW.



I then used many of the 5klm roadside markers to each town/locality to continue confirmation of the GPS.
Again, over a couple of thousand klms, and at least 20-30 different sets of those 5klm distance markers both in Vic, but mainly in NSW, the GPS metering was confirmed.
My conclusion is that those 5klm distance markers are accurate enough to gauge the car's odometer accuracy too.