Speedometer Error Calculator

Your speedometer counts driveshaft or wheel rotations and converts them to speed using the factory tire diameter. Upsizing wheels or tires increases overall diameter — each rotation covers more ground, so the speedometer reads lower than your true speed.

Actual speed ≈ indicated speed × (new diameter ÷ stock diameter). A +3% taller tire means the needle shows 60 mph when you are really doing about 61.8 mph. The reverse applies for smaller tires.

Example: stock 26.0 in overall diameter, new 27.0 in (+3.8%). At an indicated 65 mph, actual speed ≈ 67.5 mph. Pair with our tire size calculator to get diameters from metric tire codes.

The odometer drifts by the same percentage as the speedometer, which is the more expensive consequence. A 3.8% taller tire under-records roughly 380 miles in every 10,000, so scheduled service arrives late, lease mileage counts low against you, and a mileage-based warranty expires later than the dashboard suggests.

Factory speedometers are deliberately optimistic. UN ECE Regulation 39, which governs much of the world's vehicle certification, allows an indicated speed to read high but never below true speed — so most cars already show a few percent more than you are doing. Fitting taller tires eats into that built-in margin before it starts under-reading.

Tread wear moves the number too. A new tire loses several millimetres of tread depth over its life, shrinking rolling diameter by around 1 to 1.5% between new and worn out, which partly offsets an upsize. Measure or specify diameter at the tire's current condition, not its brochure figure, and always at the placard pressure.

GPS is the practical reference, with caveats. Sample on a straight, level road at steady cruise, since satellite speed is derived from position change over time and will lag through curves, hills, and urban canyons. Compare several readings rather than one, and note that GPS measures ground speed, so a headwind does not affect it.

Some vehicles can be recalibrated properly. Many trucks and SUVs accept a dealer or aftermarket tire-size parameter change that corrects speedometer, odometer, and shift points together; others need an inline correction module. This tool estimates the error only — it does not change what the car reports.

GPS speed sampling works best on straight, level road at steady cruise — hills, curves, and urban multipath can jitter readings by a few mph momentarily.

Replace tires with a different overall diameter and speedometer error changes — recalculate after any wheel or tire size upgrade.