Fitment
Tire Size Calculator
Enter a tire size — width, aspect ratio, and rim diameter — to see its overall diameter, and optionally compare a second size to see the speedometer difference. All values are the ones you enter.
Current tire
New tire (optional — to compare)
Tire dimensions
Enter the current tire's width, aspect ratio, and rim diameter above.
in overall diameter
- New tire diameter
- in
- Difference
- %
- At an indicated 60 mph, true speed
- mph
Calculated from the values you entered — this is your own figure, not data or a valuation from VINHistoryLabs.
How to compare tire sizes
- Read your tire size from the sidewall, e.g. 225/45R17 — that is width 225, aspect ratio 45, rim diameter 17.
- Enter the width in millimetres, the aspect ratio, and the rim diameter in inches for your current tire.
- Optionally enter a second (new) tire size to compare against it.
- The tool computes each overall diameter and, if you entered two, the percentage difference and the effect on your speedometer reading.
Frequently asked questions
- How is overall tire diameter calculated?
- The sidewall height is the width times the aspect ratio (a percentage). The overall diameter is the rim diameter plus twice that sidewall height, converting millimetres to inches as needed.
- How does tire size affect my speedometer?
- A speedometer is calibrated for a specific tire diameter. A larger overall diameter makes the true speed higher than shown; a smaller diameter makes it lower. The tool shows the difference between two sizes.
- What does 225/45R17 mean?
- 225 is the tread width in millimetres, 45 is the aspect ratio (sidewall height as a percent of the width), R means radial construction, and 17 is the wheel (rim) diameter in inches.