Calculating Nominal Tire Diameters

High-profile tires use customary units, and the aspect of height to width is fixed at 90%. So the tire distance from ground to rim is .90 of the width, in the case below 11 inches.

To compute the approximate overall size of an 11R22.5 tire:  

1. Sidewall height: 11 inches (tire width) X 90% (tire profile) = 9.9 inches

2. Wheel diameter = 22.5 inches

3. Two sidewall sections: 9.9 X 2 = 19.8 inches  

19.8 + 22.5 =  42.3 inches (there are some difference for tread depth, but this is a nominal number)  

Low-profile tires use a more modern metric sizing.  Their width is depicted in millimetres, and the aspect ratio varies so is always noted. This is written as width in millimetres/aspect ratio. Below 295 is the width and 75 is the ratio.

To calculate the approximate overall size of a 295/75R22.5 tire:

1. Convert the metric width to inches (295 divided by 25.4 = 11.6 inches) 

2. Sidewall height: 11.6 (tire width) X 75% = 8.7 inches

3. Wheel diameter: 22.5 inches

4. Two sidewall sections: 8.7 X 2 – 17.4 inches

17.4 + 22.5 = 39.9 inches (again, there are some difference for tread depth)  

Low-profile tires are always slightly wider and a bit lower to the ground than their high-profile counterparts. So now you know