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post #4 of (permalink) Old 21st April 2006
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Speedos are allowed a discrepancy from -0% to +10%, so manufacturers usually set them to over-read slightly, so that they don't have to get the instrument tolerances too tight. It could be that the speedo originally overread by nearly 10% and the slight difference in rolling radius of the non-factory spec wheels has made this marginally worse.

But also be aware that GPS only gives an accurate reading (usually to within 1mph) on a straight and level road. If you're going uphill or downhill, then GPS will under-read to a certain degree (trigonometry comes into it, so don't ask me for an exact figure) so if you were comparing while going up and down an Alp () then that could explain part of the difference.
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