WHAT do those numbers on a wheel mean? Well, using 17 x 8.5 6/139 P7 as an example, the first number is the wheel diameter in inches (so 17 inches); the second number indicates the wheel width in inches (in this case, 8.5 inches). We’ve covered the importance of a wheel’s diameter, but the wheel’s width is important because the wider the wheel, the more horizontal grip will be provided.

The next number (6) indicates the number of wheel studs, followed by the Pitch Circle Diameter (139), which is the diameter, in millimetres, of the circle which passes through the centre of all the studs, wheel bolts or wheel rim holes.

The next digits (P7) explain the offset of the wheel, from the hub mounting surface to the centre of the wheel. Also measured in millimetres, the wheel offset results in a positive, negative or zero offset – in this case, positive 7mm.