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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.
Currently in Horatio's garage:
2010 Laguna III Initiale.