Tire Size Calculator
Decode a P-Metric tire size (like 225/45R17) into overall diameter, sidewall, circumference, and revolutions per mile — and compare two sizes.
What is Tire Size Calculator?
The tire size calculator decodes a P-Metric tire size (like 225/45R17) into actual dimensions — overall diameter, sidewall height, and how many times the tire turns per mile. Compare two sizes to see the speedometer error from plus-sizing or down-sizing.
Formula
Sidewall (in) = (width × aspect / 100) ÷ 25.4
Overall diameter (in) = wheel + 2 × sidewall
Circumference (in) = diameter × π
Revs per mile = 63,360 ÷ circumference (63,360 in = 1 mile)
Speedometer error: larger tire = speedo reads low. Actual speed = indicated × (new diameter ÷ stock diameter).
Worked example
A 225/45R17 tire:
- Sidewall = 225 × 0.45 / 25.4 ≈ 3.99 in
- Overall diameter = 17 + 2 × 3.99 ≈ 24.97 in
- Circumference ≈ 78.45 in
- Revs/mile ≈ 807
Plus-sizing to 245/40R18 gives a diameter of 25.72 in — your speedometer will read about 2.9% low (58.3 mph when GPS shows 60).
How to use this calculator
- Read your tire size off the sidewall. Example:
P225/45R17— 225 width, 45 aspect ratio, 17 wheel. - Enter those three numbers in the first row.
- To compare a replacement size, enter the candidate size in the compare row. Stay within ±3% of the stock diameter to keep the speedometer legal and avoid rubbing.
Frequently asked questions
What does the "R" in 225/45R17 mean?
R stands for radial construction — the dominant modern ply layout. D means diagonal/bias ply (mostly trailer tires). B is bias-belted (rare today).
How much diameter change is safe?
Most manufacturers recommend staying within ±3% of the original tire diameter. Bigger changes can trip the ABS, stability control, and odometer, and may cause the tire to rub the fender or suspension at full steering lock.
Why are my miles-driven off after changing tires?
Your odometer counts tire rotations. Bigger tires cover more ground per rotation, so the odometer under-reports mileage. A 3% larger diameter = 3% fewer miles shown than actually driven.
What is "plus sizing"?
Keeping overall tire diameter nearly the same while moving to a larger wheel (and shorter sidewall). A "+1" keeps diameter within ~3%, giving crisper handling but a harsher ride.