Compare tyre sizes and see the impact on speedometer and ride height.
205/55R16
225/45R17
Diameter Difference
+2.4mm
+0.4%
| Metric | Current | New |
|---|---|---|
| Diameter (mm) | 631.9 | 634.3 |
| Diameter (in) | 24.9 | 25.0 |
| Sidewall (mm) | 112.8 | 101.3 |
| Circumference | 1985mm | 1993mm |
| Revs/Mile | 811 | 808 |
At an indicated 60mph, actual speed: 59.8mph
The numbers on your tyre's sidewall, like '205/55R16', are not just jargon; they are a precise code that defines the tyre's dimensions and characteristics. Understanding this code is essential for vehicle safety, performance, and finding the right replacement. This Calculator Site tool helps you compare two different tyre sizes to see how the changes might affect your vehicle.
The three main numbers refer to the Width (in millimeters), the Aspect Ratio (the sidewall height as a percentage of the width), and the Rim Diameter (in inches). Even a small change can alter your car's ride height, gearing, and crucially, its speedometer accuracy. For more motoring calculations, check out our Fuel Cost Calculator.
The 3% Rule
As a general rule, you should aim to keep the new tyre's overall diameter within 3% of the original tyre. A larger difference can negatively impact your vehicle's handling, braking systems (like ABS), and transmission. It may also void your insurance. Our calculator highlights this difference in red if it exceeds the 3% threshold.
Changing tyre size affects more than just how your car looks. A larger diameter tyre will cover more ground with each revolution, causing your speedometer to read lower than your actual speed. Conversely, a smaller tyre will make the speedometer read higher. You can also explore our Car Tax Calculator for other vehicle-related costs.
+1.5%
Larger tyre makes speedo read low
1.6mm
Minimum legal tread depth in the UK
7mm
Typical tread depth of a new tyre
How to use this Calculator Site tool
Enter the width, aspect ratio, and rim size for your car's current tyre and the new tyre you are considering. The calculator will instantly show you the difference in diameter, sidewall height, and circumference. It also calculates the impact on your speedometer, showing your true speed when your dashboard reads 60mph.
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