Tyre size

Tyre size is one of the markings on a tyre’s sidewall that is easiest for a motorist to understand, and it usually looks something like this:

195/65 R15

A marking like this on the sidewall of a car tyre means the following: the tyre in front of you has a width of 195 mm, an aspect ratio of 65% of the width (that is, 195 x 0.65 = 126.75 mm), an inner (rim) diameter of 15 inches, and is made using radial tyre construction (that’s right, R does not stand for radius, as some motorists think, but marks a radial tyre).

It is precisely these parameters (except for the radial construction) that determine the overall geometry of the wheel and let you judge whether a tyre suits you in terms of its physical size. You also need to bear in mind that the tyre width (195) is directly linked to the rim width, so if you are buying tyres to fit onto wheels you already have, make sure the tyre width matches the rim width. Likewise, if you are planning to buy wheels for tyres you already have, you need to take this into account when choosing the wheels. How do you find out the rim width?

The easiest way to find out what the rim width should be for any tyre size is to use our site’s tyre size calculator. You will notice that for every tyre width the calculator offers not one exact rim width but a permitted range of values. Ideally, your rim width should sit somewhere in the middle of that range, but even if it is at one of the extremes, that is nothing to worry about - it may just make the tyre fitter’s job of fitting the tyre a little harder.

Whether a tyre is radial tells the consumer very little today, because the vast majority (if not all) of car tyres made in the world are radial. Even so, for general knowledge you can read in more detail about what radial tyres are and how they differ from cross-ply ones.