Tire Size Calculator (Diameter & Speedometer Correction)
Compare two tire sizes — diameter, circumference, revolutions per mile, and speedometer correction percentage.
What this calculates
Changing tire size — even slightly — changes your effective gearing, speedometer reading, and ground clearance. This calculator compares any two tire sizes (in standard 225/45R17 notation) and tells you the diameter difference and the speedometer reading at any given true speed.
Formula & how it works
Tire diameter = wheel_diameter + 2 × (width × aspect / 100 / 25.4). Sidewall height (mm) = width × aspect ÷ 100. Convert to inches: ÷25.4. Total diameter in inches = wheel + 2 × sidewall_inches. Speedometer reading at 60 mph = 60 × (new_diameter ÷ old_diameter).
Worked example
Stock 225/45R17 vs upgrade 245/40R17. Stock diameter = 17 + 2 × (225 × 0.45 ÷ 25.4) = 17 + 7.97 = 24.97 in. Upgrade = 17 + 2 × (245 × 0.40 ÷ 25.4) = 17 + 7.72 = 24.72 in. Difference = 0.25 in smaller. Speedometer reading: 60 mph indicated = 59.4 mph actual (you'd be going slightly slower than displayed).
Frequently asked questions
Why does tire size affect speedometer?
Speedometer assumes stock tire diameter. Larger tires cover more distance per revolution, so you're actually going faster than indicated. Smaller tires = slower than indicated.
How much variation is safe?
Generally ±3 % from stock diameter to stay within ABS, traction control, and transmission tolerances. Beyond that, sensors get confused. Off-road and lift kits often need ECU reprogramming to handle large changes.
What's the aspect ratio number?
The middle number (45 in 225/45R17) is sidewall height as a percentage of tire width. Lower aspect = shorter sidewall, sportier handling, rougher ride. Higher aspect = taller sidewall, better cushioning, slightly slower steering response.
Plus-sizing?
Increasing wheel diameter while keeping total tire diameter the same. E.g., going from 17" wheels with 225/45 tires to 18" wheels with 225/40 tires — same outside diameter, shorter sidewall, looks more aggressive. Doesn't affect speedometer.