Tire Size Calculator
225 mm
45
17 in
Overall Diameter24.97 in
Circumference78.45 in
Sidewall Height101.3 mm
Revs per Mile808
The Tire Size Calculator turns a standard P-metric tire code — section width in millimetres, aspect ratio, and rim diameter in inches — into the physical dimensions that actually matter on the road. It computes the sidewall height, overall tire diameter, circumference, and how many revolutions the tire turns per mile. Those figures let you compare two sizes and gauge effects on ride height, gearing, and speedometer accuracy.
Formula
D = 2·(W·AR/100)/25.4 + rim; C = π·D; revs/mi = 63360 / C
- W
- Section width in millimetres
- AR
- Aspect ratio (sidewall height as a percent of width)
- rim
- Wheel rim diameter in inches
- D
- Overall tire diameter in inches
How it works
- Enter the three numbers from a tire size like 225/45R17: section width (225 mm), aspect ratio (45%), and rim diameter (17 in).
- Sidewall height is width × aspect ratio ÷ 100, in millimetres. Overall diameter adds two sidewalls (converted to inches at 25.4 mm per inch) to the rim diameter.
- Circumference is diameter × π (in inches), and revolutions per mile is 63,360 inches per mile divided by the circumference.
Worked example
Find the dimensions of a 225/45R17 tire.
- Sidewall height = 225 × 45 ÷ 100 = 101.25 mm.
- Overall diameter = (101.25 × 2) ÷ 25.4 + 17 = 7.97 + 17 = 24.97 in.
- Circumference = π × 24.97 = 78.45 in; revs/mile = 63,360 ÷ 78.45 ≈ 808.
Overall diameter 24.97 in, circumference 78.45 in, sidewall 101.25 mm, ~808 revolutions per mile.
Frequently asked questions
- What do the numbers in a tire size like 225/45R17 mean?
- The 225 is the section width in millimetres, the 45 is the aspect ratio (sidewall height as a percentage of the width), and the 17 is the rim diameter in inches. The R indicates radial construction.
- How does tire size affect my speedometer reading?
- A taller tire travels farther per revolution, so the speedometer reads lower than your true speed; a shorter tire reads higher. Compare the revolutions-per-mile of two sizes to estimate the error.
- What is revolutions per mile and why does it matter?
- It is how many full turns the tire makes to cover one mile (63,360 inches divided by the circumference). It drives both speedometer/odometer accuracy and effective gearing.
- How much size difference is safe when changing tires?
- Keeping overall diameter within roughly 3% of the original generally preserves speedometer accuracy and clearance. Larger jumps can affect handling, gearing, and may rub against the bodywork.