Originally Posted by mdhawkin

I often see people inquiring about metric tire sizes and what they are equivalent to in inches. For those who do not know how to convert a metric tire size to an inch equivalent I wanted to post this. Hopefully you can find it helpful.

Let’s use a common stock tire found on the F150:
275x65x18
275 = width in mm
65 = aspect ratio, this is the side wall height as a percentage of the tires width
18” = rim size in inches

To convert this tire to standard units (inches) use the following steps:
(NOTE: I round to the 100th place)
1. Convert the width in mm to inches: 275/25.4 = 10.83" (25.4 mm = 1 inch)
2. Calculate the sidewall height: 10.83 x .65 = 7.04" (don’t forget the decimal as this is a %)
3. Calculate final tire height: (total sidewall = sidewall x 2) 7.04" x 2 = 14.08". Now add your total sidewall height to your rim size, 14.08"+18" = 32.08"

This means a 275x65x18 is ~32.08x10.83x18

Hope this helps.....