Thread: Speedo vs GPS

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?

Originally Posted by DeanoH

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.

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.

Originally Posted by 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?

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.

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.

Originally Posted by 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

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.

Originally Posted by Narangga

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.

Originally Posted by DeanoH

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.

Originally Posted by BMKal

..... 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