Speedometer Correction
Enter original and new tire sizes to find speedometer correction at any speed.
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How Tire Size Affects Your Speedometer
Your speedometer is calibrated for your factory tire size. When you change tire sizes, the speedometer becomes inaccurate because the number of wheel revolutions per mile changes.
Larger tires travel farther per revolution, so the speedometer reads lower than actual speed. This calculator tells you your true speed at any indicated reading.
Speedometer error when changing tire size: % error = (new diameter − old diameter) / old diameter × 100. If new tire is 2% larger, displayed speed is 2% lower than actual — the car shows 60 mph but you're actually going 61.2 mph. Conversely, a smaller tire makes the speedometer over-read.
Example: replacing 225/55R17 (OD=679.3mm) with 245/55R17 (OD = 429.4 + 2×134.75 = 698.9mm). Error = (698.9−679.3)/679.3 × 100 = 2.9%. At a true 60 mph, speedometer reads 60 × (679.3/698.9) = 58.3 mph. You'd be 1.7 mph faster than displayed — potentially a speeding concern.
Most vehicles tolerate +/−3% tire diameter change without recalibrating the ECU. Beyond this, the ABS, traction control, and odometer all develop compounding errors. Many modern vehicles allow speedometer recalibration through OBD-II diagnostic software or dealer programming when official wheels/tires are changed.
Staggered fitments (wider rear tires than front) must calculate separately for front and rear. The speedometer reads from a single axle (usually front on FWD, rear on RWD) — only the axle providing the wheel speed sensor signal needs diameter-matched tires for accurate speedometer reading.
See also: Tire Size Calculator · Wheel Offset Calculator