making sense of tyre sizes

I am having to change my jeep tyres for the first time and I am now facing the universal problem of what tire?

Well, my jeep is having a tyre size of 31X10.50R15. This is not the conventional tyre sizes in mm. This sizing convention was new to me. So I dug in to figure out that this means. This is how it works

Let’s take the tyre size 31x10.50R15.
31 – Stands for the height of the tire in inches

10.5 – width is the tire in inches

15 – Stands for the rim size again in inches

So there is nothing to it. So how do you find the height of the sidewall?

Rim + (2 x sidewall) = height
Sidewall = (height – rim) /2
In this case its 8 inches ((31-15)/2)

But how does this convert to the conventional tire sizes? To do this we need to understand the how conventional tire sizes are read. Let’s take a similar tire 265/75R15

265 - Stands for the tire width in mm

75 – Which really means 75% is the tyre aspect ratio. This is basically the ratio of sidewall divided by width. Therefore to find the absolute sidewall height

Aspect ratio = sidewall / width
Sidewall = aspect ratio * width = 75% * 265 = 198.75 mm (7.82 inches)

15 - Stands for the RIM size in inches

Download this excel sheet to try the conversion with other sizes.