Reading Tire Sizes
A tire size like 225/45R17 means: 225mm tread width, 45% aspect ratio (sidewall height is 45% of the width), and 17-inch rim diameter. The overall tire diameter determines speedometer accuracy, ground clearance, and ride quality.
When changing tire sizes, it's important to stay within 3% of the original diameter to maintain accurate speedometer readings and avoid clearance issues.
Tire Diameter Formula
Common Tire Sizes
| Tire Size | Diameter (in) | Circumference (in) | Revs/Mile |
|---|---|---|---|
| 205/55R16 | 24.9 | 78.1 | 812 |
| 225/45R17 | 25.0 | 78.5 | 808 |
| 235/40R18 | 25.4 | 79.7 | 795 |
| 245/45R18 | 26.7 | 83.8 | 756 |
| 265/70R17 | 31.6 | 99.3 | 638 |
Frequently Asked Questions
How much tire size difference is acceptable?
Stay within 3% of the original overall diameter. Going beyond this can cause speedometer inaccuracy, affect ABS/traction control systems, and potentially void warranties.
Does tire size affect gas mileage?
Yes. Wider tires create more rolling resistance, reducing MPG. Taller tires change the effective gear ratio, which can slightly improve highway MPG but reduce city efficiency. The effect is typically 1-3%.
Can I put different size tires on my car?
Yes, within limits. Plus-sizing (larger rim, lower profile) is common and maintains the same overall diameter. Always check that the new size doesn't rub on fenders or suspension components.