Tire Size Comparison
Compare tire diameters and circumferences
How to read a tire size (e.g. 225/65R17)
225
Width (mm)
Tread width in millimeters
65
Aspect Ratio (%)
Sidewall height = 65% of width
R17
Rim (inches)
R = Radial, 17" wheel
Format: Width/Aspect Ratio R Rim (e.g., 225/65R17)
Diameter Difference
+2.1%
✓ Within acceptable range
Quick Facts
- •Format: Width/Aspect R Rim
- •Keep diameter within 3% of original
- •Larger diameter = speedometer reads low
Reading Tire Sizes
Tire sizes like 225/65R17 mean: 225mm wide, 65% aspect ratio (sidewall height as % of width), R for radial, 17" rim diameter. Changing tire sizes affects speedometer accuracy, ride height, and handling. Stay within 3% diameter difference from OEM specs.
Understanding the tire size comparison
A tyre size like 225/45R17 encodes width in millimetres, sidewall height as a percentage of that width, and rim diameter in inches. Changing any one of them changes the overall diameter, which changes the speedometer reading, the gearing, and sometimes the clearance.
Worked examples
Working out the overall diameter of 225/45R17
- 1Sidewall height = 225 mm × 0.45 = 101.25 mm.
- 2There are two sidewalls: 101.25 × 2 = 202.5 mm.
- 3Rim diameter 17 in = 431.8 mm.
- 4Total diameter = 202.5 + 431.8 = 634.3 mm, which is 24.97 in.
225/45R17 stands about 634 mm, or 25.0 inches, tall.
Checking whether 235/40R18 is a valid plus-one size
- 1Sidewall = 235 × 0.40 = 94 mm, doubled is 188 mm.
- 2Rim 18 in = 457.2 mm.
- 3Diameter = 645.2 mm, which is 24.9 in.
- 4Compared with the original 634.3 mm that is a 1.7 percent increase, inside the recommended 3 percent limit.
235/40R18 is a valid plus-one upgrade with only a 1.7 percent diameter change.
Reading a tyre sidewall
| Marking | Meaning |
|---|---|
| 225 | Section width in millimetres |
| 45 | Aspect ratio, sidewall height as a percent of width |
| R | Radial construction |
| 17 | Rim diameter in inches |
| 94 | Load index, 94 equals 670 kg per tyre |
| V | Speed rating, V equals 240 km/h |
| DOT 3021 | Made in week 30 of 2021 |
Speedometer error from a diameter change
Indicated speed of 100 km/h on the original size.
| Diameter change | Actual speed | Odometer effect |
|---|---|---|
| -3% | 97 km/h | Reads 3 percent more distance than travelled |
| -1% | 99 km/h | Slightly over-reads |
| 0% | 100 km/h | Correct |
| +1% | 101 km/h | Slightly under-reads |
| +3% | 103 km/h | Reads 3 percent less distance than travelled |
Frequently asked questions
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This calculator is for educational and reference purposes only. Always verify critical calculations independently.