Tire Size Calculator
Compare a current tire size with a possible replacement. A different overall diameter changes your true speed, your odometer, and your gearing, even when the rim size looks close.
Current tire (for example 205/55R16)
Replacement tire (for example 225/45R17)
Your results
Worked example
Swapping 205/55R16 for 225/45R17:
- Current: sidewall 205 × 0.55 = 112.75 mm; diameter = (16 × 25.4) + (2 × 112.75) = 631.9 mm; circumference 1,985 mm
- Replacement: sidewall 225 × 0.45 = 101.25 mm; diameter = (17 × 25.4) + (2 × 101.25) = 634.3 mm; circumference 1,993 mm
- Difference: +0.38%, small enough to be a non-issue
- True speed at an indicated 100 km/h: 100 × (634.3 ÷ 631.9) = 100.4 km/h
How this calculator works
The first number on a tire is its width in millimetres, the second is the sidewall height as a percentage of that width, and the number after R is the rim diameter in inches. A common guideline is to stay within about 3 percent of the original diameter.
Common questions
What happens if the difference is over 3 percent?
Speedometer and odometer error grows, gearing and shift points change, and on some vehicles the ABS and stability systems can be confused by mismatched sizes. Clearance can also disappear at full steering lock or over bumps. Staying within about 3 percent avoids most of this.
Does this affect my odometer and warranty?
Yes, in proportion to the difference. A tire 2 percent larger records about 2 percent fewer kilometres than you actually travel. That also means your recorded fuel economy looks slightly better than it really is.
Estimates only. Results are provided for planning purposes and are based on nominal size figures; actual tire dimensions vary by brand and model. Confirm fitment, load rating, and clearance with a tire professional before buying. Mississippi Maple accepts no liability for purchase decisions made using this calculator.