Tyre Comparison Calculator
Compare two tyre sizes side by side. Check overall diameter, sidewall height, circumference, and how much a size change will affect your speedometer accuracy.
Tyre Sizes
Enter two tyre sizes to compare them side by side
Comparison Results
Diameter, circumference, and speedometer impact
Enter both tyre sizes above and click Compare Tyre Sizes to see the full side-by-side comparison.
Understanding Tyre Size Codes
A tyre size such as 225/45R17 contains everything needed to calculate its physical dimensions and rolling circumference.
Width
The first number (e.g. 225) is the tyre’s section width in millimetres, measured from sidewall to sidewall.
Aspect Ratio
The second number (e.g. 45) is the sidewall height as a percentage of the width. Lower numbers mean a shorter, stiffer sidewall.
Construction & Rim
“R” denotes radial construction, and the final number (e.g. 17) is the wheel diameter in inches that the tyre is designed to fit.
Overall Diameter
Overall diameter = rim diameter + (2 × sidewall height). This determines circumference, rolling distance, and speedometer accuracy.
Tyre Size Tolerance Guide
A general guide to how much an overall diameter difference is likely to affect your vehicle.
| Diameter Difference | Rating | Speedo Impact | Notes |
|---|---|---|---|
| 0% – 1% | Ideal | Negligible | Essentially unnoticeable in daily driving |
| 1% – 3% | Acceptable | Minor | Generally considered a safe plus-size swap |
| 3% – 5% | Caution | Noticeable | May affect ABS, traction control, and clearance |
| 5%+ | Not Recommended | Significant | Risk of rubbing, gearing, and sensor calibration issues |
Tyre Comparison FAQ
Everything you need to know about comparing tyre sizes and how they affect your vehicle.
A tyre size such as 225/45R17 breaks down as follows: 225 is the tyre width in millimetres, 45 is the aspect ratio (sidewall height as a percentage of width), R indicates radial construction, and 17 is the wheel diameter in inches.
A speedometer is calibrated for the rolling circumference of the original tyre. If you fit a tyre with a different overall diameter, the wheel rotates a different number of times per mile, so the speedometer and odometer will read slightly higher or lower than your true speed and distance.
Plus sizing refers to fitting a larger diameter wheel with a lower profile (shorter sidewall) tyre while keeping the overall tyre diameter close to the original. This preserves speedometer accuracy and ride characteristics while changing the wheel’s appearance.
Most tyre professionals recommend keeping the overall diameter difference between an original and replacement tyre within about 3%. Differences beyond this can affect speedometer accuracy, ABS and traction control calibration, gearing, and ground clearance.
The sidewall height is the distance from the wheel rim to the outer edge of the tread. It is calculated by multiplying the tyre’s width by its aspect ratio percentage, for example a 225/45 tyre has a sidewall height of 225 × 0.45 = 101.25mm.
Yes. Wider tyres typically increase rolling resistance, which can reduce fuel economy slightly. Taller tyres can also change effective gearing, which may affect engine RPM at a given speed and influence fuel consumption.