Tire Size Inches

How to read a tire size

A metric tire size states three things: section width in millimetres, sidewall height as a percentage of that width, and wheel diameter in inches.

The size code 285/70R17 with each field called out: section width in millimetres, aspect ratio as a percentage of that width, construction letter, and wheel diameter in inches.285/70R17Section width, mmAspect ratio, % of widthConstructionWheel diameter, in

Reading the sidewall

  1. The first number is the section width in millimetres, measured sidewall to sidewall.
  2. The second number is the sidewall height as a percentage of that width, not a measurement.
  3. The letter is the construction — R is radial, which is nearly everything sold today.
  4. The last number is the wheel diameter in inches, and it is the one number already in inches.
  5. Two sidewalls plus the wheel diameter gives the overall diameter.

A worked example

For 285/70R17: the sidewall is 199.5 millimetres, two of them are 15.71 inches, and adding the 17 inch wheel gives 32.71 inches overall.

How this is worked out

sidewall height in millimetres = section width × aspect ratio ÷ 100

overall diameter in inches = wheel diameter + (section width × aspect ratio × 2) ÷ 2540

section width in inches = section width in millimetres ÷ 25.4

circumference in inches = π × overall diameter

revolutions per mile = 63360 ÷ circumference in inches

The divisor is 2540, not the 2550 some converters publish. Two sidewalls in inches are 2 × width × aspect ÷ (100 × 25.4), and the rounding costs about 0.07 inches at 34 inches.