Gizmo said...
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Virtually all cars that I've been in with GPS in them they seem to match the speedo without much error. So are you also saying that GPS is not accurate?
No. GPS is quite accurate, which is part of the problem.
If your results are different then you've either been lucky, or not checked enough cars.
My experience over the past 20 years was that all cars had an error of around 4%. So that to do 100 kph you have the speedo on 104. All the cars I have owned were very close to this. I could do a random comparison by slowing to match speeds with cars on the freeway and find they were doing 100 on the speedo but around 96 by the GPS. At 60kph the difference between GPS and speedo was negligible. This leads me to believe that the 4% figure at 100kph used to be a standard.
I have done my checks using up to 3 GPS at a time. The GPS were all within 1 kph of each other (different brands, types, purposes and one built-in car system)
I bought a new car last year and the difference was much greater and applied at all speeds. So to do 60kph I have to drive about 65 and to do 100 kph I do about 107. That is when I read about the ADR. The 5% +2 thing comes from car owners forums, and a phone call to the Subaru dealer, so I cannot vouch for the accuracy of that.
The problem with people driving off their GPS is you get law abiding people doing 100 by their speedo and other law abiding people doing 100 by their GPS which is 5-7kph faster. If the speedo people and the GPS people start to get a little grumpy then you get road rage.
Here's a related issue, the odometer and the speedometer on my car are calibrated separately. If you travel at exactly 110 kph for exactly one hour then the odometer only shows 104km travelled. The manufacturer has to have an accurate odometer reader so they can give km based warranties. You could sue them for fraud if the odometer read high and they tried to reject a warranty claim based on km travelled.