What’s in a number?
Let’s take a popular tyre among off-road enthusiasts: The BF Goodrich 265/65 R17 All Terrain T/A KO2 120S. The first figure and letter group on the sidewall is its size. The 265 means the tyre is 265 mm wide; 65 is the thickness of the wall as a percentage of the width, in other words the thickness is 65% of 265 mm (265 mm x 65% = 172 mm). The R17 refers to the inside radius of the tyre (in inches), in other words, this tyre fits a 17” wheel rim.
So why do manufacturers use millimetres and inches? It’s a long story but, in short, American legislation is more concerned with the inside radius (in inches), while European tyre manufacturers also focus on the tyre’s width (in mm). As America is an important market for tyre manufacrers – and being one of only two countries that still uses inches (the other is Liberia) – manufacturers are forced to keep the inches measurement.
All Terrain T/A KO2 is simply the name and model version of the tyre.
The final numbers and letter on a tyre wall are the load and speed indexes. In this case, the 120 load index determines that this tyre can carry a weight of 1 400 kg per wheel and the S means that the tyres are limited to a safe top speed of 180 km/h. Below is a chart to explain the ratings: