# Re: Formula for tire size conversions

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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

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

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