Calculating Nominal Tire Diameters
- Z-factor Consultation
- Aug 10, 2020
- 1 min read
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
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