All cars are setup about 10% over reading. The comment about servicing your car more often as a result is not correct since the odometer is correctly setup. If you travel at 60mph on the speedo for 10 mins, you wont cover 10 miles.

All big manufacturer cars will do this no matter how new. My brothers 56 plate audi 4a underread by exactly 10%.

Satnavs speed calculations are accurite to 0.001% I beleive and use a doppler shift from multiple satalites to work our your exact velocity (velocity not being the same as speed).

JAmes.