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
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.