Speedometer Correction
Speedometer error from a GPS reading or a known correction factor: true speed at each indicated speed, the reading for a wanted true speed, the odometer effect.
The correction factor and error percentage, a table of true speed at common indicated readings, the indicated reading that gives a target true speed and the odometer distance factor.
Example: 100 km/h indicated against 94 km/h on GPS is a factor of 0.94: the speedometer reads 6.4 % high, 110 km/h true needs 117 indicated, and the odometer records 6.4 % more distance than driven.
One factor,
the whole dial.
How a GPS check gives the correction, why a tyre change is exactly proportional, and what the odometer does.
The check
Drive at a steady speed on a straight, level road with a good satellite fix and note the speedometer and the GPS speed at the same moment. GPS speed is true ground speed averaged over a second or so; the ratio f = GPS ÷ indicated is the correction factor and true speed = indicated × f at every reading, because a speedometer (and the vehicle speed sensor it reads) is very close to linear. The error 1 ÷ f − 1 is how much the dial over-reads; cars are normally calibrated to over-read slightly rather than ever under-read.
Tyres and gearing
The sensor counts revolutions, so a tyre with a different rolling circumference or a different final drive scales indicated speed by exactly the ratio of circumferences (or of ratios) — enter that as the factor in the second mode to see the new error without a test drive. The odometer usually shares the sensor and records 1 ÷ f of the real distance: a dial reading 6 % high logs 6 % too many kilometres.
Tolerances and limits
Legal tolerances (UNECE R39: indicated never below true and at most 10 % + 4 km/h above) are labelled, not applied. Tyre wear changes the circumference by a percent or so; speed changing during the sample, a poor fix, or a reading taken on a bend give large errors, which the page flags above 25 %. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- f = GPS speed ÷ indicated speed; true = indicated × f; indicated error = 1/f − 1; odometer records 1/f of the real distance; legal tolerances (e.g. UNECE R39) named, not applied
Last reviewed 21 September 2026. How results are checked: How we verify.