Tire Size Calculator
Enter a tire size like 225/65R17 to see diameter, sidewall height and circumference, and compare two sizes for speedometer error.
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About the Tire Size Calculator
A tire size such as 225/65R17 contains three numbers: the section width in millimeters (225), the aspect ratio (65, meaning the sidewall is 65 % of the width) and the wheel diameter in inches (17). This tire size calculator converts them into overall diameter, sidewall height and circumference, and compares two sizes so you can see how a swap changes your speedometer reading.
When you change wheels or tires, keep the overall diameter within about 3 % of the original to avoid speedometer, ABS and gearing problems.
How to use it
- Type your tire size exactly as printed on the sidewall (for example 225/65R17).
- Optionally type a second size to compare.
- Press Calculate to see diameter, sidewall, circumference and speedometer difference.
How to read a tire size
| Part | Meaning (225/65R17) |
|---|---|
| 225 | Section width in millimeters |
| 65 | Aspect ratio: sidewall height is 65 % of the width |
| R | Radial construction |
| 17 | Wheel diameter in inches |
Tire diameter formula
Overall diameter (mm) = wheel diameter (in) × 25.4 + 2 × (width × aspect ratio ÷ 100). For 225/65R17: 17 × 25.4 + 2 × 146.25 = 724.3 mm (28.5 in).
Speedometer error after a tire change
If the new tire is larger, the car travels farther per wheel revolution, so the speedometer reads lower than your true speed. If it is smaller, the speedometer reads higher.
Frequently asked questions
How do I calculate tire diameter?
Wheel diameter in inches × 25.4 plus twice the sidewall height, where sidewall = width × aspect ratio ÷ 100 (all in mm).
What does 225/65R17 mean?
225 mm wide, sidewall 65 % of the width, radial construction, 17-inch wheel.
How much tire size difference is safe?
Most manufacturers recommend staying within about 3 % of the original overall diameter.
Will bigger tires change my speedometer?
Yes. Larger diameter means your real speed is higher than the speedometer shows. The comparison above estimates the difference.
Can I fit a wider tire on the same wheel?
Each tire has an approved rim width range. Check the tire maker chart and your vehicle clearance.
Last reviewed: November 2026.
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