How the correction works
Fuel used is measured correctly; only distance is wrong. The real distance is the odometer distance multiplied by the ratio of the tire circumferences:
- Real distance = odometer distance × (new circumference ÷ original circumference)
- Real MPG = measured MPG × the same ratio
- Real L/100 km = measured L/100 km ÷ the same ratio
Worked example
A pickup came with 265/70R17 (31.6 in) and now runs 285/70R17 (32.7 in), 3.49 % bigger. The trip computer says 18.0 MPG, but the truck really managed 18.0 × 1.0349 = 18.6 MPG. After 1,000 miles on the odometer it has actually covered about 1,035 miles.
What changes real fuel use
- Weight: larger all-terrain tires can weigh 10–20 lb more each.
- Rolling resistance: aggressive tread and lower pressures increase it.
- Gearing: taller tires lower engine RPM, which may help or hurt depending on the engine — see the gear ratio calculator.
- Aerodynamics: lift kits and wider tires add drag at highway speed.
Frequently asked questions
Do bigger tires really lower MPG?
Two things happen. Bigger, heavier, wider tires usually do increase real fuel use (more weight, rolling resistance and drag). But they also make the odometer under-read, which makes the measured MPG look worse than it is. This calculator removes the second, measurement-only effect.
Why is my trip computer MPG wrong after a tire change?
The car counts wheel revolutions to measure distance. With taller tires each revolution covers more ground than the car thinks, so the recorded distance — and the MPG calculated from it — is too low by the same percentage.
Does it work with L/100 km?
Yes. Choose L/100 km and the correction is applied the other way round: the true figure is lower (better) than the measured one when the tires are bigger.
Can I get the speedometer recalibrated?
On many US trucks a dealer or a plug-in programmer can set the new tire size, which corrects the speedometer, odometer and MPG display together. Otherwise apply the correction yourself.