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Are the speedometer and odometer in my Toyota Prius over-reading?

I've just bought a new 61-plate Toyota Prius which has a completely electronic dashboard display. Checking the speedo against a SatNav, it showed that at a constant 70mph indicated (held by cruise control on a level motorway) the SatNav was showing 64mph. I rechecked this the following day against a different, hand-held GPS and that also showed 64mph against an indicated 70mph. So that car speedo seems to be over reading by 10 per cent, give or take a decimal point.

I am genuinely shocked by this. I think it raises some important questions. 1. Which should I believe - the speedo or the GPS? 2. If the GPS, is it possible to get the speedo recalibrated and made accurate? 3. Does this not mean that distance is being incorrectly recorded? That is, if it shows I have travelled 110 miles, have I only travelled 100 miles in reality? If so, over the life of the car, it will record literally thousands of miles in error, severely affecting trade in value.

Asked on 25 February 2012 by BH, via email

Answered by Honest John
For the 10,000th time (or something like that), speedometers are allowed to over-read up to the equivalent of about 10 per cent but are not allowed to under read at all. Consequently, they nearly all over-read by between 3 per cent and 10 per cent and this small margin of error saves countless drivers from getting speeding tickets. Of course, it can also hold you up by 5mph in a 50mph contra-flow. Yes, you are correct that odometers can also over-record. On a regular '300 mile' journey I get a variation to between 290 and 310 miles. I don’t think you can get it recalibrated.
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