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Speedometer Correction Calculator
Find speedometer error after gear or tire changes
Speedometer correction calculates true road speed after a tire-size change, and the axle ratio that restores the original gearing.
Formula: Actual speed = indicated × (new diameter ÷ old diameter) · Restoring gear = gear × (new ÷ old)
What this number means
This calculates how far off your speedometer reads after a rear gear or tire size change, and the corrected true speed for a given indicated speed. It applies to street-driven race cars and tow vehicles alike.
What to change next
Use the percentage to correct the speedometer gear, electronic calibration or simply your driving until it is fixed — the error is proportional, so it grows with speed. Recheck it whenever tires or gears change again.
Worked example
Swapping a 26 in tire for a 28 in: an indicated 60 MPH is actually ≈ 64.6 MPH — the speedometer reads about 7.7% low.
Related calculators
Read the guide: Gear Selection Guide →
Building a Jeep or 4x4 too? The same gear math drives your crawl ratio. RegearReady — our sister site — turns it into regear decisions: Correct your speedometer on RegearReady.