It is with hesitation that I contradict someone who describes themself as "Winner of 3 CCUK awards. Sex god. Master Debater.", but Dublet's answer above is misleading.
There is one sensor (on the gearbox) that counts revolutions. The signal is used by the speedo and the odometer. If the speedo is reading 10% fast, then the odometer is reading 10% fast too.
The law allows for speedos to be up to 10% fast, but never slow. To be safe, most manufacturers aim for 5% fast. Wheel size has no relation to the speedo, the speed/mileage is derived from the revolutions and the outside diameter/circumference of the tyres. Bear in mind that tyre wear affects accuracy, the diameter reduces with wear. A worn tyre will read about 2% faster than a new one.