Tyre Size Calculator
Compare your current tyres with a new size and see the change in diameter, sidewall height and speedometer accuracy before you buy.
Compare Two Tyre Sizes
Enter the three numbers from each tyre sidewall, for example 205/55 R16. We’ll show how the new size differs and what your speedometer will really be telling you.
Tyre Size Comparison
Check overall diameter and speedometer error in seconds.
How it Works
Comparing tyre sizes in four steps
Enter Your Current Size
Type the width, aspect ratio and rim diameter shown on the sidewall of your existing tyres.
Enter the New Size
Add the size you are considering, such as a larger alloy wheel with a lower-profile tyre.
Set a Speed
Choose a speedometer reading, like 70 mph, to see what your true speed would be on the new tyres.
Review the Difference
See the change in diameter, sidewall height and speedometer accuracy, plus a quick suitability note.
Common Tyre Sizes & Dimensions
Calculated dimensions for popular UK tyre sizes, plus a guide to reading the numbers on your sidewall.
| Tyre Size | Sidewall Height | Overall Diameter | Circumference | Revs per Mile |
|---|---|---|---|---|
| 175/65 R14 | 113.8 mm | 583.1 mm | 1,831.8 mm | 879 |
| 195/65 R15 | 126.8 mm | 634.5 mm | 1,993.4 mm | 807 |
| 195/55 R16 | 107.3 mm | 620.9 mm | 1,950.6 mm | 825 |
| 205/55 R16 | 112.8 mm | 631.9 mm | 1,985.2 mm | 811 |
| 215/55 R17 | 118.3 mm | 668.3 mm | 2,099.5 mm | 767 |
| 225/45 R17 | 101.3 mm | 634.3 mm | 1,992.7 mm | 808 |
| 225/40 R18 | 90.0 mm | 637.2 mm | 2,001.8 mm | 804 |
| 245/45 R18 | 110.3 mm | 677.7 mm | 2,129.1 mm | 756 |
| Reading 205/55 R16 91V | 205 = width in mm. 55 = sidewall height as a % of width. R = radial. 16 = rim diameter in inches. 91 = load index. V = speed rating. | |||
| Diameter formula | Overall diameter (mm) = (width × aspect ratio ÷ 100 × 2) + (rim inches × 25.4) | |||
Tyre Size FAQ
Answers to the most common questions about reading tyre sizes, changing wheel sizes and speedometer accuracy.
The first number, 205, is the tyre width in millimetres. The second, 55, is the aspect ratio: the sidewall height as a percentage of the width. The letter R means radial construction, and 16 is the wheel rim diameter in inches. You will usually also see a load index and speed rating after the size, such as 91V.
Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in millimetres. Double it, then add the rim diameter converted to millimetres (inches × 25.4). For 205/55 R16 that is (205 × 0.55 × 2) + (16 × 25.4) = 631.9 mm.
Often yes, but the new size should keep the overall diameter within roughly 2 to 3 percent of the original so the speedometer, gearing and stability systems are not affected. You also need to check wheel clearance, rim width compatibility, and that the load index and speed rating meet or exceed the vehicle manufacturer’s requirements. Always confirm with your vehicle handbook or a qualified tyre fitter.
The speedometer counts wheel rotations and assumes a fixed tyre circumference. A larger overall diameter means the car travels further per rotation, so your true speed is higher than the speedometer shows. A smaller diameter does the opposite and makes the speedometer read too high.
The size is printed on the sidewall of the tyre. It is also usually listed in the vehicle handbook and on a label inside the driver’s door frame or fuel filler flap.