Tire Size Speedometer Calculator
See how a tire size change affects your speedometer reading and true speed.
Changing tire size throws off your speedometer, because the system was calibrated for the original tire's circumference. This calculator shows your true speed and the error introduced by a different tire diameter.
Speedometer Error Formula
If you fit taller tires, each wheel revolution covers more ground, so you travel faster than the speedometer reports. Shorter tires do the opposite. The same effect skews your odometer reading.
How to Use This Calculator
- Enter the original tire diameter (what the car was calibrated for).
- Enter the new tire diameter.
- Enter the speedometer reading to see true speed.
Worked Example
Why Tire Size Changes Your Speedometer
A speedometer counts driveshaft or wheel rotations and assumes each turn covers a fixed distance — the original tire's circumference. Fit a taller tire and each rotation covers more ground, so you're really going faster than the needle shows. Fit a shorter tire and the opposite happens. The same error scales your odometer, so a tire change quietly affects recorded mileage too.
Correcting Speedometer Error
Small diameter changes (under ~3%) are often left as-is, but larger swaps — common on lifted trucks and off-roaders — should be corrected. On most modern vehicles this means reprogramming the speedometer calibration for the new tire size; older vehicles may need a different speedometer gear. Always calculate the error first so you know how far off you are.
Speedometer Error by Tire Change (from 26")
| New diameter | Indicated 60 MPH | Error |
|---|---|---|
| 24" | 55.4 MPH true | over-reads 7.7% |
| 28" | 64.6 MPH true | under-reads 7.7% |
| 31" | 71.5 MPH true | under-reads 19% |
Frequently Asked Questions
A larger tire travels farther per revolution, so the car actually moves faster than the speedometer shows. Smaller tires make it read high.
Multiply the indicated speed by the ratio of new diameter to old diameter. Taller tires give a true speed higher than indicated.
Yes. The same error applies — taller tires make the odometer under-count miles, while shorter tires over-count.
Going from 26 to 28 inches is about a 7.7% increase, so at an indicated 60 MPH you'd truly be doing about 64.6 MPH.
Often yes — many modern cars allow a recalibration, and aftermarket programmers or gear changes can correct it for larger tire fitments.