How to use Tire Size Comparison Calculator
The tire size calculator decodes the three-number tire code (e.g. 225/45R17) into diameter, sidewall height, circumference and section width, then compares two tire sizes side by side. It also calculates speedometer error so you know how much your indicated speed will differ from actual speed when fitting non-OEM tires.
- Enter the original (stock) tire size in the format Width/Aspect-Ratio/Rim (e.g. 225/45/17).
- Enter the new tire size you are considering.
- Review diameter, sidewall height, circumference and section width for both.
- Check the speedometer error percentage and whether the new tire fits.
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Reading a tire size code
A tire code like 225/45R17 means: 225 = tread width in mm; 45 = aspect ratio (sidewall height as a percentage of width, so 45% × 225 = 101.25 mm sidewall); R = radial construction; 17 = rim diameter in inches. Overall diameter = rim diameter (in mm) + 2 × sidewall height. Circumference = π × overall diameter.
| Size | Overall diameter | Sidewall height | Section width |
|---|---|---|---|
| 205/55R16 | 24.9" | 4.5" | 8.1" |
| 225/45R17 | 25.0" | 4.0" | 8.9" |
| 235/50R18 | 27.3" | 4.6" | 9.3" |
| 255/55R18 | 28.0" | 5.5" | 10.0" |
| 265/70R17 | 31.6" | 7.3" | 10.4" |
Speedometer error and safe sizing
A larger-than-stock overall diameter means the car travels further per wheel rotation, so the speedometer reads low (you are going faster than indicated). A smaller diameter reads high. A ±3% difference is considered acceptable by most standards; ±5% affects odometer accuracy and can cause ABS/traction-control calibration issues. Always verify physical clearance (fender, suspension) before fitting significantly different tire sizes.
Worked examples
225/45R17 tire
Inputs: 225 × 0.45 = 101.25 mm sidewall
Result: Diameter ≈ 17 + 2 × 3.99 in ≈ 25.0 in
265/70R17 (truck / all-terrain)
Inputs: 265 × 0.70 = 185.5 mm sidewall
Result: Diameter ≈ 17 + 14.6 in ≈ 31.6 in
Speedo error: 225/45R17 → 235/45R17
Inputs: diameter 25.0 in vs 25.33 in
Result: +1.3% — speedo reads ~1.3% slow (within ±3%)
Glossary
- Aspect ratio
- Sidewall height as a percentage of section width; lower = wider, sportier handling.
- Section width
- The widest point of the tire in millimetres.
- Overall diameter
- The total outer diameter of a mounted, inflated tire.
- Plus sizing
- Fitting a larger-rim tire with a lower-profile sidewall to maintain overall diameter.
Related reading
Frequently Asked Questions
Why use Tire Size Comparison Calculator?
- Decodes the full tire size code into diameter, sidewall height and circumference in one step
- Calculates speedometer error precisely so you know indicated speed versus actual road speed
- Flags when a proposed tire change exceeds safe diameter tolerance for ABS and traction control calibration
- Compares original and replacement tire sizes side by side to confirm physical fitment compatibility
Common use cases
- Check whether a set of 235/45R18 tires will clear the wheel arch without rubbing on full lock
- Calculate the speedometer correction needed after fitting larger all-terrain tires on a truck
- Find the nearest-matching metric tire size to replace an unusual OEM specification on an imported car
- Verify that a set of dedicated winter wheels preserves the same overall diameter as the summer setup
- Determine how a lower-profile tire on a wider rim changes ride height and handling characteristics
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