Tools you might need next
Calculate engine horsepower from torque and RPM or estimate from quarter-mile time and weight. Convert between HP, kW, and brake horsepower units.
Estimate 0-60 mph acceleration time from horsepower, vehicle weight, and drivetrain loss. Model realistic launch performance for tuning and comparison.
Calculate fuel injector size from target horsepower, BSFC, and duty cycle. Size injectors for naturally aspirated, turbo, and supercharged builds.
Tire diameter equals rim diameter plus twice the sidewall height. Sidewall height is section width multiplied by aspect ratio percentage.
Diameter = Rim + 2 × (Width × Aspect / 100)Smaller diameter tires rotate more times per mile, effectively raising gearing. RPM at a given speed increases proportionally to diameter decrease.
Revs/mile = 63,360 / (π × Diameter inches)Speedometer error equals the percentage difference in diameter. A tire 3% larger reads 3% slow (actual speed higher than indicated).
Error % = (New diameter − Old diameter) / Old diameter × 100Updated: July 2026
Stock diameter 24.97 inches vs plus-size 25.67 inches (+2.8%). Speedometer reads 65 mph when actual speed is 66.8 mph. Minimal gearing change.
265/70R17 (31.6") to 285/75R17 (33.8") adds 2.2 inches diameter (+7%). Speedometer reads 60 when actual is 64.2 mph. Regear or recalibrate required.
235/35R19 (25.5") to 225/45R17 (25.0") on smaller winter wheels: −2% diameter, speedometer reads slightly fast. Acceptable for seasonal use.
Always match overall diameter within 3% when changing wheel size. A wider tire on a larger rim may still have the same diameter if aspect ratio is reduced proportionally.
AWD systems require all four tires within 2–3% diameter. One worn or different-size tire causes differential binding and expensive drivetrain damage.
Changing tire size affects speedometer accuracy, gearing, ground clearance, and fender fitment. This calculator compares overall diameter, revolutions per mile, and speedometer error between stock and replacement tire sizes.