Tire Size Calculator
Parse a tire size like 225/45R17 to get sidewall, diameter, circumference, and revolutions per mile. Compare two sizes for diameter delta and speedometer error.
Compare two tire sizes
Tire A
225/45R17
- Sidewall
- 101.3 mm
- Diameter
- 634.3 mm (24.97")
- Circumference
- 1992.7 mm
- Revs / mile
- 808
Tire B
245/40R18
- Sidewall
- 98.0 mm
- Diameter
- 653.2 mm (25.72")
- Circumference
- 2052.1 mm
- Revs / mile
- 784
Comparison (B vs. A)
- Diameter difference
- +18.9 mm (+2.98%)
- Actual speed at indicated 60 mph
- 61.79 mph
- Speedometer error
- +2.98%
Frequently Asked Questions about the Tire Size Calculator
How do I read a tire size like 225/45R17?
The first number (225) is the section width in millimeters. The second number (45) is the aspect ratio, meaning the sidewall height is 45% of the width. R means radial construction, and 17 is the rim diameter in inches.
What is the sidewall height and how is it calculated?
Sidewall height is the distance from the wheel rim to the outer edge of the tire. The formula is width times aspect ratio divided by 100. For 225/45R17 that gives 225 * 45 / 100 = 101.25 mm.
Why does the aspect ratio matter for ride and handling?
A lower aspect ratio (like 40) means a shorter, stiffer sidewall, giving sharper steering response but a harsher ride. A higher aspect ratio (like 60) gives a taller sidewall, better comfort, and more sidewall flex during cornering.
How do different tire sizes affect my speedometer?
Speedometers read off wheel rotation, not actual road speed. A larger overall diameter means fewer revolutions per mile, so your actual speed is higher than indicated. The calculator shows the percent error at an indicated 60 mph.
How much speedometer error is acceptable when changing tire sizes?
Most carmakers and tuners aim to keep the new overall diameter within about 3% of the original. Beyond that, the speedometer, odometer, ABS, and traction control calibration can all drift far enough to fail an inspection in some regions.
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