To make sure you’re getting the best performance from your driveline, a few calculations are necessary.
The final gear ratio of your transmission works with your rear axle ratio & tire size to make all of this happen.
To dial in the best efficiency, these all need to be considered for an upgrade.
You can use a tape measure and provide us with the overall tire height
( 26″ for instance is a typical standard height. )
STEP BY STEP INSTRUCTIONS TO FIND YOUR TIRE SIZE
If you want to know if a particular spare tire cover will fit, or are trying to find out how much clearance you would have when switching to another tire/wheel combo, there is an easy way to find out.
Simply read the numbers from the sidewall of your current vehicle or trailer tire.
Common Tire Size Parts
The most common tire sizes will have three parts, just like the 205/75-R15 tire pictured above.
This number will tell you three things about the tire and wheel combination that you are trying to fit.
The First Number
The number 205 represents the section width of the tire, in millimeters.
This is the measurement of the tire at its thickest point.
If you want to convert centimeters to inches, divide the number of centimeters by 2.54. 1 Inch is about equal to 2.54 centimeters.
Example: 20.5 cm / 2.54 = 8.070 in
A 205/75-R15 tire would have a section width of 8.070 inches.
The Second Number
The number 75 refers to the sidewall aspect ratio.
This will help you find out the height of the sidewall, from rim to tread.
This is also called section height.
The second number in tire size indicates the ratio between tire width and height.
Example: 8.070 in x 0.75 = 6.053 in
A 205/75-R15 tire would have a section height that is 75 percent of 8.070 inches.
The Third Number
The last number, 15 in our example, represents the diameter, in inches, of the wheel on which the tire is mounted.
Determining Total Diameter
Now that you know the section width, section height, and rim size of the 205/75-R15 tire and wheel, you are able to determine the total diameter.
The section height, or sidewall on the tire, is 6.053 inches.
Multiply this by 2 to determine the amount of tire extending out on both sides of the wheel.
Imagine drawing a straight line across the tire and wheel, directly across the center.
Example: 6.053 in x 2 = 12.106 in
Add the combined section heights of the tire (12.106 in) to the wheel size to get the combined diameter of the tire and wheel.
15 in + 12.106 in = 27.106 in
Section Height = Section Width x Aspect Ratio
Tire and Wheel Diameters
Total Diameter = (Section Height x 2) + Wheel Diameter