Thread: Speedo vs GPS

Originally Posted by AK83

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.

Originally Posted by Graeme

..... 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.

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.

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

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!

Originally Posted by AK83

My conclusion is that those 5klm distance markers are accurate enough to gauge the car's odometer accuracy too

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

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.

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.

Originally Posted by 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!

Originally Posted by Graeme

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....
Cheers,
Chris