August 16, 2020
Do you know how to read a tyre size? A standard metric car/4WD tyre size is something like 205/65R15 or 245/70R16. The 205 and the 245 are the width of the tread in mm. The 65 and 70 are a percentage of the width equaling the sidewall so 65% of 205mm is 133mm making the sidewall 133mm high and 70% of 245 being 171.5mm. Next the R is for radial. The last number is the diameter of the rim in inches, we’ll convert this to mm to work out the overall diameter of the tyre, 15″ is 381mm and 16″ is 406mm. So for the first size 205/65R15 we times the sidewall x 2 (for top and bottom) so 133mm x 2 = 266mm + the rim diameter so 266mm + 381mm = 647mm so your tyre is 205mm wide with an overall diameter (height) of 647mm. Back to the 245/70R16 we have the sidewall of 171.5mm, let’s times that by 2 = 343mm plus the rim height of 406mm = 749mm overall diameter. So 245/70R16 is 245mm wide and 749mm overall diameter.
An imperial size is different again, let’s use 31×10.50R15 as an example. It’s all in inches. 31″ is the overall diameter, 10.50″ is the width and 15″ is the rim size (same as the metric).