The size of the tyre can be marked in inches (imperial) or millimeters (metric). For example, 16.9R34 is an inch size. The equivalent size marked in mm is 420/85R34. Although written differently, they are exactly the same size.

Example One

The tyre size can be composed with two sets of numbers, for example: 12.4-24.

1. Number 12.4 is the tyre width in inches

2. Number 24 is the rim diameter in inches

Example Two

A tyre size can also be composed of three sets of numbers, for example: 16×6.50-8, 31×15.50-15.3 or 25×12.00-12.

16×6.50-8:

1. Number 16 is the tyre diameter in inches

2. Number 6.50 is the tyre section width in inches

3. Number 8 is the size of the rim in inches

650/65R38:

650 = Width of the tyre in mm (side-wall to side-wall, not the pattern size)

65 = Aspect Ratio – Height percentage of width (650mm x 0.65 = 422.5mm)

R = for Radial Tyres

38 = Diameter in Inch

Handy Hint

To assist with easy conversion from inches to mm (in radial tractor tyres) simply multiply the width by 25mm (25.4 to be exact).

Example Three

11.2R24 (11.2 x 25mm = 280mm)

Standard profile 85 series, hence 280/85R24