Tire comparison and conversion tool
Tire Size Calculator
Compare two metric or flotation tire sizes and estimate diameter, circumference, revolutions, speedometer error, odometer correction, clearance and gearing changes.
How tire size calculations work
- Metric diameter: rim diameter × 25.4 + 2 × section width × aspect ratio ÷ 100.
- Circumference: π × calculated diameter.
- Revolutions: distance per mile or kilometer ÷ circumference.
- Corrected speed and distance: indicated value × new diameter ÷ current diameter.
- Ride-height change: half of the diameter difference.
- Effective axle ratio: original ratio × current diameter ÷ new diameter.
Example
Changing from 225/65R17 to 245/65R17 increases the calculated diameter from about 28.52 to 29.54 inches. If the speedometer reads 60 mph, the mathematical estimate is about 62.15 mph, and ride height increases by about 0.51 inch.
Important fitment limits
This calculator compares nominal dimensions; it does not approve a tire for a vehicle. Actual mounted dimensions vary by manufacturer, tread, inflation pressure and approved rim width. Diameter alone cannot confirm load index, speed rating, wheel width, offset, brake clearance, fender clearance, suspension travel, steering clearance, TPMS compatibility or legal compliance. Confirm the vehicle placard and tire/wheel manufacturer specifications before changing size.
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