Tyre size calculator
Thinking of fitting wider or bigger tyres, or replacing tyres with a different size than the original? Thistyre size calculator decodes sizes like 185/65 R15 and compares two of them:sidewall height, overall diameter, circumference and revolutions per kilometre. Most importantly, it shows the speedometer difference — your true speed when the speedometer reads 100 km/h — and warns if the change is more than the commonly recommended 3%.
How to use it
- Type your current tyre size from the tyre sidewall (e.g. 185/65 R15).
- Type the new size you're considering.
- Check the diameter difference and speedometer effect.
Example
Going from 185/65 R15 (overall 621.5 mm) to 195/55 R16 (620.9 mm) changes the diameter by just −0.1% — an almost perfect “plus one” upsize, so the speedometer stays accurate. Jumping to 205/65 R15 (647.5 mm) would be +4.2%: at an indicated 100 km/h you'd really be doing about 104 km/h. Plan fuel costs with thefuel cost calculator and check mileage with themileage calculator.
Frequently asked questions
What does a tyre size like 185/65 R15 mean?
185 is the tyre’s width in millimetres, 65 is the aspect ratio — the sidewall height as a percentage of the width (65% of 185 mm = 120 mm) — R means radial construction, and 15 is the wheel (rim) diameter in inches.
Can I fit a bigger tyre on my car?
Usually only if the overall diameter stays within about 3% of the original, so the speedometer, ABS and clearance aren’t badly affected. Wider tyres may also rub on the wheel arches. Check your vehicle manual or a tyre dealer before changing size.
How does tyre size affect the speedometer?
A larger overall diameter travels further per turn, so you go faster than the speedometer shows; a smaller one makes the speedometer over-read. The calculator shows your true speed when the speedometer reads 100 km/h.
What is “upsizing” or “plus sizing”?
Fitting a bigger rim with a lower-profile tyre, such as going from 185/65 R15 to 195/55 R16, keeping the overall diameter almost the same. It sharpens handling but gives a slightly firmer ride.