Tyre sizes

53 common tyre sizes with their real diameter, sidewall height and rolling circumference.

A tyre marking is three measurements

Take 205/55R16. The tread is 205 mm wide. The sidewall is 55% of that — 112.8 mm — and there is one at the top and one at the bottom. The rim is 16 inches, or 406.4 mm. So:

diameter = 16 × 25.4 + 2 × (205 × 0.55)
         = 406.4 + 225.5
         = 631.9 mm

The aspect ratio being a percentage is the part that surprises people. A 225 tyre at 55 is taller than a 205 at 55, because 55% of a bigger number is bigger. It also means two markings that look unrelated can describe almost the same tyre.

Why one car is offered three different sizes

205/55R16, 225/45R17 and 195/65R15 come out at 631.9, 634.3 and 634.5 mm — a spread of 0.41%. That is why a manufacturer can list all three against the same model: the wheel gets bigger, the sidewall gets shorter, and the rolling diameter stays where the speedometer, the gearing and the ABS expect it.

The rule this produces is worth carrying: go up an inch of rim, drop about 10 points of aspect ratio, and add 10 mm of width, and the diameter comes out close.

What changing the diameter does

A speedometer counts wheel revolutions and multiplies by a fixed factor, so it reads in proportion to the diameter it was calibrated for. Fit a tyre 1% larger and the speedometer reads 1% low: at an indicated 100 km/h you are doing 101. Odometer, trip computer and fuel consumption all shift by the same proportion. Most jurisdictions require a speedometer never to under-read, which is why factory tyres usually read slightly high and why replacements are kept inside about 3%.

13-inch

14-inch

15-inch

16-inch

17-inch

18-inch

19-inch

20-inch

21-inch

22-inch

Diameter is not the whole decision. A replacement also has to suit the rim width and carry the vehicle's load and speed ratings, which are the letters and numbers after the size on the sidewall. The placard in the driver's door frame is the authority for what a particular car takes.

Unit converters · Metric threads · Wire gauge