Speedometer Error Calculator

This calculator shows your real road speed, the percentage error, how far you actually travel for every 100 miles on the odometer, and your true speed at an indicated 30 mph. Enter the stock tire diameter, the diameter of the tires now fitted and any speedometer reading.

It is most useful after lifting a truck and fitting larger tires, plus-sizing wheels on a car, switching to winter tires, or simply as tires wear. The speedometer counts wheel or driveshaft revolutions and assumes the stock tire's circumference, so any change in diameter changes both speed and odometer readings by the same percentage.

If you only know tire sizes like 265/70R17, work out the diameter first – the formula is below, or use the tire size calculator. A gear change alters the same ratio; the gear ratio calculator handles axle and transmission ratios.

Distance per wheel revolution is π × diameter, so speed and distance scale directly with diameter. Real tires are slightly smaller than their nominal size under load, and new tread is deeper than worn tread, so treat results as accurate to within about 1%. Speed units do not matter – the ratio is the same in mph or km/h.

A metric size such as 265/70R17 means a 265 mm section width, a sidewall height 70% of the width, and a 17 in rim. Sidewall height = 265 × 0.70 = 185.5 mm, or 7.30 in. Diameter = 17 + 2 × 7.30 ≈ 31.6 in. Flotation sizes such as 33×12.50R15 state the diameter directly: 33 in.

In the UK, EU and other countries using UN Regulation 39, a speedometer must never show less than true speed and may over-read by no more than 10% plus 4 km/h. Manufacturers therefore usually calibrate speedometers to read slightly high from the factory, which is why a GPS often shows a lower speed than the dash.

A common guideline when changing tire sizes is to stay within about 3% of the original diameter. Beyond that, speed readings, odometer mileage, ABS and stability-control calculations, and automatic transmission shift points can be noticeably affected. Plus-sizing wheels (for example 16 in to 18 in) while keeping overall diameter close – 205/55R16 to 225/40R18 in the table – keeps the error under 1%.

• GPS check: hold a steady speed on a straight, level road and compare the dash with a phone GPS app. GPS is accurate at constant speed but lags when speeding up or slowing down.
• Mile markers: time a measured mile; 60 seconds means exactly 60 mph.
• Recalibration: many modern vehicles store tire size in the computer, which a dealer or an aftermarket programmer can update. Older vehicles may need a different speedometer drive gear.
• Odometer: recalibrating fixes the odometer too; until then, multiply readings by the odometer figure from this calculator. Fuel economy worked out from odometer miles is understated by the same percentage on taller tires – correct the miles before using the MPG calculator.