What’s in a num­ber?

Go! Camp & Drive - - 4X4-PROFESSOR -

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:

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