If you’re not sure how to read them, the size markings on a Truck and Bus Radial (TBR) tyre’s sidewall may seem little more than a meaningless and confusing jumble of numbers and letters! The fact is, however, that they contain a lot of important and useful information that is key to ensuring which tyres are right for your vehicle — and understanding this is far easier than it looks.
And there’s more good news! TBR tyres sizing is largely based on the standard metric formula for passenger and 4 x 4 tyres – with just a few technical differences. So, once you get to grips with the formula and apply it to your tyres, the numbers and letters take on a whole new meaning.
Here’s a bit of a breakdown using two examples of examples of a common TBR size markings you may encounter:
295/80 R22.5
295 equals the Section Width of the tyre (across the widest point of the tyre). In other words, it’s the width of the tyre in mm when mounted to a rim of a specified width.
80 is the Aspect Ratio or Profile – which is the height of the sidewall. This number is actually a percentage of the section width, so in this case the aspect ratio is 80% of the section width.
R means the tyre is a Radial construction — which is what most common type of tyre construction these days.
22.5 Represents the diameter of the rim (in inches) that the tyre suits and should be fitted to.
Let’s take a look at another example:
11R22.5
11 equals the width of the tyre in inches
R once again refers to Radial construction, and
22.5 is the diameter (in inches) of the rim that the tyre it suitable to be fitted to.
You may notice that unlike in the previous example, there is no aspect ratio in this tyre size. This is because for this size, the aspect ratio/height of the sidewall) is around 85 (or 85 % of the tread width) which is an accepted industry standard, so it is not necessary to have this included.
The way size markings are displayed on TBR tyres may vary slightly according to the manufacturer. Fortunately, however, global industry standards ensure that once you’ve grasped the meaning of basic markings on one TBR tyre, you’ll be well on the road to reading and interpreting others along the way.
