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Home Tyre Calculator Help

Tyre Calculator Help

The easiest way to buy new tyres is just to drop your car at a shop and tell them “new tyres, please”.

But that’s not necessarily going to get you the best tyre for your needs, whether you’re a 4X4 tourer, tower or racetrack lover.

There’s a dizzying array of information about tyres and cars, and one online tool can’t possibly handle it all. But, it can help.

Very often people wish to change their tyre’s dimensions; 4X4 owners often want taller tyres for better offroad performance, and sports car owners often want wider tyres for looks and corner grip. This calculator helps you compare your existing vs new tyres by giving you some comparison measurements and advice.

A tyre’s size is found on the sidewall of the tyre, and it looks like this:

The numbers you’re looking at for size in the picture above are 245/70/17. The rest aren’t relevant to sizing. The “R” stand for Radial, a type of tyre construction.

And here’s a roadcar example, for 215/45/17:

So what do those numbers mean? Let’s take a look at a 235/85R16 tyre:

The first number, 235, is the tread width in millimetres. The second number, 85, is a percentage. It refers to the aspect ratio, or sidewall height. So to find the sidewall height we multiply 235 * 85% = 200mm. Confusing, but that’s the way it is.

Finally, the 16 is the rim diameter. And that’s in inches! So to convert that to mm = 16 x 25.4 = 406mm.

To find the overall diameter of the tyre it’s two sidewalls of 200mm each, and the 406mm rim so: (200mm x 2) + 460mm = 806mm. And, often tyre diameters are referred to in inches…so we’d call that a 31.7 inch tyre.

Yes, a lot of mixing and matching of units there, but that’s just the way tyres are.