Free tool
Tyre size calculator
Enter your current tyre size and the one you're considering to see the diameter difference and what it does to your speedometer, before you buy.
| Current size | New size | Difference | |
|---|---|---|---|
| Overall diameter | |||
| Sidewall height | |||
| Tread width | |||
| Circumference | |||
| Revolutions per km |
How the calculator works
A size like 205/55 R16 holds everything we need: 205 is the width in millimetres, 55 means the sidewall is 55% as tall as the tyre is wide, and 16 is the rim diameter in inches. From those three numbers we work out the overall diameter, circumference and revolutions per kilometre, then compare the two sizes.
The 3% rule
Your speedometer measures speed by counting wheel revolutions. If the new tyre has a bigger diameter, it covers more ground per turn, so you're going faster than the speedometer shows. A smaller tyre does the opposite. Within a 3% difference the error stays small and ABS and stability control work as designed. Beyond it you risk warning lights, tyres rubbing the arches and speed camera fines you didn't expect.
Common size changes in the UAE
| Car | From | To | Diameter change |
|---|---|---|---|
| Toyota Camry | 215/55 R17 | 235/45 R18 | +0.1% |
| Nissan Sunny | 185/65 R15 | 195/55 R16 | −0.1% |
| Toyota Land Cruiser | 265/65 R18 | 265/55 R20 | −0.3% |
| Nissan Patrol | 275/60 R20 | 285/50 R22 | +0.7% |
The calculator doesn't check load index or speed rating. The new tyre must match or beat the factory values on your driver's door sticker. There's more on reading sizes in our tyre size guide.
Common questions
Common questions
How do you work out tyre dimensions from the size?
Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim size in inches × 25.4 + twice the sidewall height. For 205/55 R16 that's a 112.75 mm sidewall and a 631.9 mm diameter.
What is the 3% rule?
Keep the new tyre's overall diameter within 3% of the original. Inside that band the speedometer, ABS and stability control stay accurate, and the tyre usually clears the arches.
Can I fit a wider tyre on my car?
Usually 10–20 mm wider is fine if it suits the rim width and doesn't rub the arch or suspension on full lock. We check clearance before fitting.
Does the calculator check load index or speed rating?
No. It compares dimensions only. The new tyre's load index and speed rating must match or exceed your car's factory values.
Flat tyre? A technician can be on the way in minutes.
Send your location and a photo of the tyre sidewall. We'll confirm the price and arrival time straight away — 24 hours a day.