A Beginner’s Guide To 4WD Tyre Sizes

If you’re intending on getting serious about off-road & your 4x4, then you gotta learn to talk the talk before you can walk the walk (or you’ll be literally walking the trail trying find someone to help you out of a rut!) And since a 4x4 is only as good as the tyres it sits on, we’re going to give you a quick rundown on off-road tyre calculations.

Your tyre size can be calculated in metric (millimetres) or imperial (inches). When measuring a tyre, the width of the tread, the rim size and size of the side wall are taken into account.

You’ll also want to memorize the below tyre terms too which will help with your calculations (or are just generally good to know):

  • Sidewalls: sides of the tyres.
  • Track or Tread: rubber that covers the edges of the tyres.
  • Footprint: part of the tyre that maintains contact with the road.
  • Bead: the portion of the tyre that helps it stay connected to the rim.
  • Profile: the aspect ratio, the height of the sidewall.

Example:  if a tyre’s width is said to be 285 it means this figure is in millimetres (285mm). If you see the diagram to right, take it to mean the rim’s length. The rim’s diameter is given in inches. Finally, 75 is the side wall’s height (profile) which has to be used as a percentage of the tyre’s width.

 

Calculating Tyre Diameter

If you want to calculate the full diameter of the tyre, use the following calculation: in the example above, the tyre is 285mm wide. The sidewall is simply 75% of 285mm (285 x 0.75) which is 213.75mm high. This isn’t the full diameter, rather the heigh of the tyre.

To get the full diameter you need to add the rim size. In the example, this is 16″ (406.4mm). So the total tyre diameter is 213.75mm x 2 (sidewall x 2) + 406.4mm (rim size) = 833.9mm. Now when talking tyre diameter, most people & experts talk in inches – which = 32.83 inches.

Now you may have seen tyre measurements not resembling anything like this one, that’s because some tyres are only measured in inches. So for example, your tyre might be 33x11.5R16. In this case, 33 inches is the total diameter of the tyre, 11.5 is the width of the tyre and R16 is the rim size.

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