Key takeaways
- A tire size like 225/45R17 = width (mm) / aspect ratio (%) / rim (in).
- Overall diameter = rim + 2 × sidewall height.
- Aspect ratio is the sidewall as a percent of width, not a fixed height.
- Revolutions per mile = 63,360 ÷ circumference — it's what drives your speedo and odometer.
How to read a tire size
A tire size like 225/45R17 lists three things: the section width in millimeters (225), the aspect ratio as a percent of that width (45), and the rim diameter in inches (17). The "R" just means radial construction. From those three numbers you can derive every dimension that matters.
The aspect ratio matters because it's a percentage: a 45 on a 225 tire is a different sidewall height than a 45 on a 285 tire.
Worked example: 225/45R17
Sidewall = 225 × (45 ÷ 100) ÷ 25.4 = 3.99 in. Overall diameter = 17 + 2 × 3.99 = 24.97 in. Circumference = π × 24.97 = 78.4 in, so revolutions per mile = 63,360 ÷ 78.4 ≈ 808.
How aspect ratio changes diameter (225 width, 17″ rim)
| Size | Sidewall | Overall diameter |
|---|---|---|
| 225/40R17 | 3.54 in | 24.09 in |
| 225/45R17 | 3.99 in | 24.97 in |
| 225/50R17 | 4.43 in | 25.86 in |
| 225/55R17 | 4.87 in | 26.74 in |
Changing sizes the right way
Stay within about ±3% of your stock overall diameter so your speedometer, odometer, and ABS stay accurate and the tire still clears the fender and suspension. To compare a candidate against your current size, use the tire size comparison tool; to see exactly how a size change shifts your speed reading, use the speedometer error calculator.
Frequently asked questions
How do I read 225/45R17?
225 = section width (mm), 45 = aspect ratio (% of width), R = radial, 17 = rim diameter (in).
How do I calculate tire diameter?
Diameter = rim + 2 × sidewall, where sidewall(in) = width(mm) × (aspect/100) / 25.4. A 225/45R17 ≈ 24.97 in.
What does the aspect ratio mean?
It's the sidewall height as a percent of section width — 45 means the sidewall is 45% of 225 mm.
How many revolutions per mile does my tire turn?
63,360 ÷ circumference. A 24.97″ tire turns about 808 revs/mile.
Will a different size change my speedometer?
Yes — a larger diameter reads slow, a smaller one reads fast. Stay within ~3% of stock.
Section width vs tread width?
Section width (225 mm) is the widest point of the inflated tire; tread width (the contact patch) is narrower and isn't in the printed size.
Tire size notation follows the standard P-metric system used across the industry — see Tire Rack's tire-size reference. The 63,360 inches-per-mile and 25.4 mm-per-inch conversions are exact.
Last reviewed June 2026