Key takeaways
- A size like 225/45R17 = width (mm) / aspect ratio (%) / rim (in).
- 225 = section width in mm; 45 = sidewall is 45% of that width; R = radial; 17 = rim in inches.
- Overall diameter = rim + 2 × sidewall height.
- Keep replacement sizes within ~3% of stock diameter so the speedometer and clearance stay right.
Reading 225/45R17, number by number
A tire size is a compact spec sheet. Each field is a real measurement, just in mixed units (millimeters and inches) for historical reasons.
| Part | Means | In 225/45R17 |
|---|---|---|
| 225 | Section width (mm) | 225 mm ≈ 8.86 in wide |
| 45 | Aspect ratio (% of width) | sidewall = 45% of 225 = ~101 mm |
| R | Construction | Radial |
| 17 | Rim diameter (in) | fits a 17-inch wheel |
Turning the size into dimensions
The number most people actually want is the overall diameter — how tall the whole tire stands. Two short steps get you there:
For 225/45R17: sidewall = 225 × 0.45 ÷ 25.4 = 3.99 in, so overall diameter = 17 + 2 × 3.99 = 24.97 in. The tire size calculator does this instantly and also gives circumference and revolutions per mile.
Why diameter matters when you change tires
Your speedometer, odometer, and ABS are all calibrated to the factory tire's diameter. Go too far off and your speedo reads wrong and the tire may rub. The rule of thumb is to stay within about ±3% of stock diameter. To check a new size against your current one, use the tire size comparison tool, and to see exactly how a change shifts your speed reading, the speedometer error calculator.
Frequently asked questions
What do the numbers on a tire mean?
225/45R17 = section width (mm) / aspect ratio (% of width) / R for radial / rim diameter (in).
Where do I find my tire size?
On the sidewall, the driver's door-jamb placard, or the owner's manual. The placard shows the factory-recommended size.
What does the aspect ratio tell me?
Sidewall height as a percent of width — lower (40) is shorter and sportier, higher (65) is taller and softer-riding.