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post #3 of (permalink) Old 20th November 2005
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First of all, it's impossible to judge a car's speed by 'feel.' Some just naturally feel faster than others at the same road speed. Generally speaking, a larger car will feel like it's going slower than a smaller one, but there are no hard and fast rules.

With a few specialist exceptions, speedos all over-read. The reason being that the law insists that they must not under-read, but may over-read by up to 10%. The sensible thing to do, therefore, is aim for an over-read by about 5%. In any case, it's never possible to be 100% accurate as there will always be small variations in rolling radius caused by incorrect tyre pressures and even tyre wear.

The speedo on my last car didn't agree with TomTom, and the new one is even further out. I don't know whether that's down to differences in calibration, or the difference in rolling radius of the higher profile tyres on smaller wheels (the radius for the 16" wheels and tyres is marginally smaller than for the 17".)

And the new car doesn't even agree with itself. The digital readout for the cruise/limiter reads about 2mph less than the speedo at 70mph. TomTom, in turn, tends to read about 1mph below what the cruise display says. The only one I'm inclined to believe is TomTom, but even that's only accurate to 1mph when driving at a steady speed in a straight line and on level ground.
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