TripRank
Tyre Size and Your Speedometer Accuracy
Changing your tyre's diameter alters its rolling circumference, directly affecting your speedometer's accuracy and potentially leading to significant speed discrepancies.
Speed & accuracy
Why indicated speed and true speed differ, and what the regulations actually require.
Why your speedometer reads high, and why GPS doesn't
Car speedometers are legally required to over-read. UN Regulation No 39 sets the exact allowance — up to 10% plus 4 km/h — and explains the gap between your dashboard and a GPS speed app.