Tyre Size Calculator

Use Tyroola's tyre calculator to find substitute tyre sizes for your vehicle if you're looking for a change.

It will help you avoid speedometer errors since its accuracy is directly affected by tyre size. Upsizing or fitting taller / larger tyres will reflect a slower speedometer reading than your actual speed.

Look at your existing tyres and enter their size into the calculator. Do the same for another set you want to compare them with.

To find alternative tyres, simply click on each size to see what Tyroola has available.

You can use it for a wide variety of tyres including street, racetrack, trailer, spare, off-road and even extreme mud-terrain tyres.

1. How do I read a tyre size code like 265/65R17?
Break it down: 265 is the width in millimetres, 65 is the profile (sidewall height as a percentage of that width), and 17 is the rim diameter in inches. So 265/65R17 is a 265mm-wide tyre with a sidewall 65% of that width, on a 17-inch rim. It's a universal code, printed right on your sidewall, nothing to measure.

2. Can I fit a slightly bigger tyre than my car's original size?
You can, within reason. Keep the overall diameter within about 3% of standard so you don't affect handling, braking or the speedo. Go much bigger and you risk it rubbing the guard or suspension. Easiest move is to run the sizes through a calculator first and see the difference before you commit.

3. Why does my speedo read differently after changing tyre size?
Because your speedo works off wheel rotations. Fit a taller tyre and it turns fewer times per kilometre, so the speedo reads slower than you're actually going, which is worth knowing given how strict speed enforcement is here. Smaller tyres do the reverse and make it read faster than your real speed.

4. What happens if I go too far outside my original diameter?
A few things, none good. Your brakes and ABS are tuned around the original wheel-and-tyre combo, so a big change can throw braking off. It can also mean the tyre rubbing the guard or suspension through corners and over bumps. That's exactly why the rule of thumb is to stay within about 3% of the original diameter.

5. Is a tyre size calculator enough to guarantee it'll fit?
It's a great starting point, comparing diameter, sidewall height and width so you know how two sizes stack up. But it's not a guarantee: guard clearance, suspension geometry and each car's tolerances vary even on identical sizes. Treat it as a planning tool, then confirm the final fit with your fitter before you order.