Wallace Racing

BACK to Auto Math

This is a page to calculate some of the things that are affected by tire diameter. All you have to do is change the different tire size parameters and the JavaScript does the rest. For a quick explanation of the different parameters in the tire code, click here. The reason for having two colums is so that it is easy to compare the parameters of two different tires at the same time. This way it is easy to see the difference between say a 255/50-17 and a 315/35-17 without having to re-run the calculations which by the way are the current and future rear tire sizes for my Impala SS.

If you are coming here from the driveshaft critical speed page and want to predict the driveshaft speed, just enter a 1 in the transmission gear ratio box. That means the engine is going the same speed as the driveshaft. Enjoy.

Inputs
Section width (millimeters):
Aspect ratio:
Wheel diameter (inches):
Outputs
Tire height in inches:
Tire rollout in inches:
Wheel rpms at mph:
Engine rpms with rear gear and transmission gear ratio:

FORMULAS:

tire height = wheel diameter + (2 X (section width / 25.4) X (aspect ratio / 100))

rollout = tire height X PI

wheel rpm = (vehicle speed / 60) X (5280 / (rollout / 12))

engine rpm = wheel rpm X rear gear ratio X transmission gear ratio

255/50-17

255	Section Width	Total width of the tire at the sidewalls in millimeters. This is measured from the outside of the sides of the tires when mounted and not the tread or rim width.
50	Aspect Ratio	Percentage of the section width that is the sidewall height. For this example the 255mm section width multiplied by an aspect ratio of 50 gives a sidewall height of 127.5mm
17	Wheel Diameter	Wheel diameter at the tire mounting surface in inches.