Tire Size Calculator (Formula)

Updated: Jan 2

I decided to create an interactive calculator, instead of using the equations below. Still, please feel free to view the information below to better understand what is happening here.  Enjoy.

I’ll be adding tire width conversions and TireA >> TireB height comparisons soon.

Enter your tire info below to see how tall in inches the overall rubber is. Example: If your tire is 225/45R15, then you would enter those numbers in order in the fields below. 225 | 45 | 15 and your height will show in the right field once you click it to refresh.

I had another method to finding these numbers, but Zafer’s equation is much more clean and requires less steps.  Thanks, Zafer!

Taken from http://audifans.com/archives/1995/07/msg00798.html

Here’s the formula for determining the diameter of a tire based on the standard size designations:

Diameter (in inches) = (2 * tire-width * aspect ratio / 25.4 ) + wheel rim size

The division by 25.4 is to convert mm to inches.

Using 205/60-15 as an example:

Tire width = 205 mm Aspect Ratio = 60 (sidewall height is 60% of tire width) or 0.6 * 205 = 123 mm Wheel rim dia. = 15 inches

So the diameter of a 205/60-15 tire is = (2*205*0.6/25.4) + 15 = 24.69 inches

I have a simple C program that calculates tire sizes and gives a comparison of the relative differences. I picked this up from an internet site a while back. If anyone wants it I can email it to them, or we can put it up on the quattro archive.

Here’s the output of that program for the tire sizes that Glenn is considering:

% tire.x 185/60-14 195/55-14 205/55-14 Specification Sidewall Radius Diameter Circumference Revs/Mile Difference 185/60-14 4.4in 11.4in 22.7in 71.4in 887 0.0% 195/55-14 4.2in 11.2in 22.4in 70.5in 899 -1.3% 205/55-14 4.4in 11.4in 22.9in 71.9in 882 0.6%

The “Difference” column lists the % difference in size compared to the firstr tire. So, either of the last two tires can be used since the difference in diameter is minimal.



Zafer Mehmood AT&T Bell Laboratories zm@mhcnet.att.com Murray Hill, NJ

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