Calculator - Speedometer Adjustment for Tire Size Change


Speedometer ratio adjustment calculator. Ever wonder how far off your speedometer is with your new bigger tires? I'm using a simple ratio calculation, so this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assuming nothing but tire size has changed). If you know specific information about your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator, it's probably a little more accurate.

Speedometer Check (in MPH)
Orig Tire Dia.
New Tire Dia.
  Speedometer: Actual:


Formula used (New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH

Calculators and Table used with permission from Mark I. Medina (