Do speedometers overstate speed and therefore artificially increase mileage?

I recently had my satellite navigation on and noticed that at 70mph on the speedo, the Sat Nav reckoned actual speed was 65mph. I know that speedos must not understate speed and it is not uncommon for them to overstate by some margin. However, if the odometer is calibrated in the same way then when I think I have done 70 miles the car will actually only have done 65 miles. If the trip meter is then also tied into the same calibration then stated economy will also be overstated.

Also, if say the average overstatement of mileage is 10 per cent, then when my odo indicates 12,000 miles the actual mileage covered will be just under 11,000 miles. So not only could I be getting my car serviced at least once more than required over say 100,000 miles, my car will also have an additional 10,000 miles of depreciation which will have an effect on residual values. Do you think that the manufacturers take this into account when calibrating speedos as it will increase their service revenue over the life of a car? Or am I being very cynical?

Asked on 22 June 2010 by Ewanr

Answered by Honest John
Satnavs are 100 per cent accurate on a level, straight road because they measure your speed by GPS across the surface of the planet. But if you are on a twisty road they will under-read because they measure your speed in a straight line. They will also be less accurate on inclines. And where they lose sight of the satellites, such as in a city with tall buildings or in a tunnel, the computer inside them merely estimates your speed. Thanks to the IAM's chief examiner Peter Rodger for this information.
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