That string of numbers on your tyre sidewall tells you everything. We decode what 225/45R17 91W actually means so you can order the right tyre every time.
Every tyre has a size code printed on its sidewall — something like 225/45R17 91W. It looks technical, but once you know what each segment means, it's straightforward. Ordering the wrong size is a common and avoidable mistake.
Breaking Down 225/45R17 91W
Here's what each number means:
- 225 — Tyre width in millimetres (from sidewall to sidewall)
- 45 — Aspect ratio: sidewall height as a percentage of width (45% of 225mm = ~101mm tall)
- R — Radial construction (the standard for modern passenger car tyres)
- 17 — Rim diameter in inches (the wheel it fits)
- 91 — Load index (maximum load per tyre; 91 = 615kg)
- W — Speed rating (maximum safe speed; W = 168mph / 270km/h)
Where to Find Your Tyre Size
The easiest place is the tyre sidewall itself. You'll also find the manufacturer's recommended size in your vehicle handbook, on a sticker inside the driver's door jamb, or on the fuel cap flap on some cars.
Can You Change Tyre Size?
Minor variations are sometimes acceptable, but changing tyre size affects speedometer calibration, handling and potentially your MOT result. Always check with a tyre specialist before going to a different size. When in doubt, match the original specification exactly.
When booking with Totally Mobile Tyres, simply enter your vehicle registration and we'll automatically identify the correct tyre size for your car.
