Tyres have four measurements to consider, while the wheels or rims they sit on have two. Rims are measured in diameter and width, according to the tyres they fit. Tyres are measured by section width, section height or profile, rim diameter and tread width. Tread width merely notes the spread of a tyre's tread as it meets the road, and doesn't factor into rim size.
Measure the rim width. This is usually anywhere from 12.5 cm (5 inches) for small passenger cars, to 23.7 cm (9 1/2 inches) for trucks and trailers.
Measure the rim diameter. Take the diameter measurement from the inside groove that the tyre bead sits on. It's easiest to measure the inner diameter of the tyre after it's removed from the wheel. If you measure the outer diameter of the rim itself, it will be almost 2.5 cm (1 inch) larger than the specific size.
Measure the tyre size by looking at the width, height and inner diameter of a tyre. The size is written on the outer sidewall. P205/40R16 is a common tyre size notation showing all these. The "P" indicates passenger car. The "205" is the width at the widest point between the sidewalls in millimetres. The "40" signifies ratio of height to width (aspect ratio), also known as profile or series. The "R" stands for radial and the "16" is the rim diameter in inches.
Match tyres to rims. Select a rim with the diameter of the tyre. Use manufacturer-approved ratings to find an accommodating width. Tyres are able to be mounted on rim widths that vary from 3.7 to 5 cm (1 1/2 to 2 inches). Tyre manufacturers specify approved width ranges.
Consider load ratings before finalising a tyre purchase.
Never mount a tyre on a rim with an unapproved width.