Speedometer after a lift
Why the speedometer starts lying
A speedometer does not count speed, it counts wheel revolutions. A bigger tyre covers more ground per revolution – so you are going faster than the needle says.
How much
The error roughly matches the ratio of the diameters.
| Change | Error | at an indicated 100 km/h |
|---|---|---|
| 33 to 35 inch | about 6 % | actually about 106 km/h |
| 33 to 37 inch | about 12 % | actually about 112 km/h |
| 35 to 37 inch | about 6 % | actually about 106 km/h |
Six percent sounds harmless. In town, an indicated 50 km/h is about 53 – and a speed camera measures the real figure, not the indicated one.
What else depends on it
- Odometer. It counts short. At twelve percent you lose roughly 12,000 km in 100,000 – that affects service intervals and resale.
- Automatic transmission. Shift points depend on engine speed relative to road speed.
- Cruise control and driver assistance. They use the same wrong signal.
- Technical approval. An inspector who notices the deviation will raise it.
The fix
A calibrator plugs into the diagnostic port and corrects the value in the control unit. You enter the new tyre size and the axle ratio, the device writes it. On most vehicles this takes minutes and needs no wiring work.
The devices are under speedometer calibration. Whether one fits your vehicle is stated on the product page – they are model and year specific.