Your speedometer does not measure speed directly. It counts how fast the wheels are turning and multiplies by a factor that assumes the original tire size. Fit a taller or shorter tire and the factor is wrong, so the display and the odometer drift away from the truth. This calculator compares the tire you started with and the one you plan to fit, and tells you the true speed, the percentage error, and how far you really travel when the odometer says 10,000.
How to use the speedometer error calculator
- Enter the original tire size — width in millimetres, aspect ratio and rim diameter — from the sidewall or the door-jamb placard.
- Enter the new tire size in the same three boxes.
- Enter a speedometer reading, such as 60 mph or 100 km/h.
The results show the true speed, the error in percent, both overall diameters, and the real distance covered per 10,000 odometer units. To check just a diameter, use the tire diameter calculator; to see all differences between two sizes at once, open the tire size comparison calculator.
Speedometer error formula
True speed = indicated speed × new diameter ÷ old diameter
Error % = (new diameter ÷ old diameter − 1) × 100
Real odometer distance = indicated distance × new diameter ÷ old diameter
Wheel speed is proportional to road speed divided by diameter, so for the same number of wheel turns the road speed scales directly with diameter. That is why a single ratio converts speed, error and odometer.
Worked example: 225/45R17 → 235/45R18
- Original diameter: 17 + 2 × (225 × 0.45 ÷ 25.4) = 24.97 in.
- New diameter: 18 + 2 × (235 × 0.45 ÷ 25.4) = 25.95 in.
- Ratio: 25.95 ÷ 24.97 = 1.0391, a 3.9% increase.
- When the speedometer shows 60 mph, the true speed is 60 × 1.0391 = 62.3 mph.
- When the odometer shows 10,000 miles you have really driven about 10,391 miles.
Reading table for this example
| Speedometer shows (mph) | True speed (mph) | Difference |
|---|---|---|
| 30 | 31.6 | +1.6 |
| 45 | 47.4 | +2.4 |
| 60 | 63.3 | +3.3 |
| 70 | 73.8 | +3.8 |
| 80 | 84.3 | +4.3 |
How much error is too much?
A change of overall diameter within about 3% of the original is the common rule of thumb for plus-sizing, because beyond that the speedometer, ABS and stability-control calibration, gearing and clearance start to suffer. In many places a speedometer must never read lower than true speed; in the United Nations vehicle regulation (UN R39) the indicated speed may exceed the true speed by up to 10% plus 4 km/h, but it must never be lower. If your new tires make the display read low — as in the example, where the real speed is higher than shown — you can easily be speeding while the display says you are within the limit. Check your speed against a GPS app on a flat, straight road to measure the actual error.
There is also a gearing effect. A taller tire raises the effective final drive ratio the other way: the engine turns slower for the same road speed, which can slightly reduce acceleration and, on some vehicles, change shift points. If you fit significantly larger tires on a truck, consider adjusting the axle gearing with the gear ratio calculator.
Sources
The size marking and diameter formula follow the ISO metric tire system. The 3% rule of thumb and the UN R39 speedometer requirement (indicated speed never below true speed) are widely cited by tire manufacturers and type-approval regulators; check the current regulation for your region.
Frequently asked questions
Why does a bigger tire make my speedometer read low?
The speedometer counts how many times the wheel or driveshaft turns and assumes a fixed distance per turn. A taller tire covers more ground per turn, so your real speed is higher than the display.
How much speedometer error is acceptable?
Most manufacturers calibrate speedometers to read slightly high, never low. A change in overall diameter of about 3% or less is generally considered safe for fitment; larger changes should be corrected with a speedometer recalibration.
Does the odometer change too?
Yes, by the same percentage. With a taller tire the odometer under-counts the real distance, which can matter for warranty mileage and resale value.
How do I fix a speedometer after changing tires?
Depending on the vehicle, options include a programmer or dealer reflash that sets the tire size, an electronic speedometer calibrator, or a different drive gear in older vehicles. Check your GPS speed against the speedometer on a flat road to find the actual error.
Does this work in km/h?
Yes. The ratio of the diameters is unitless, so the true speed comes back in the same unit you enter.