tire quick math

April 25, 1998

April 26, 1998
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tire quick math


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 quick math vs actual tire size


First the quick math to figure the diameter of a tire

Sidewall height of a General Tire "255 / 65 R 16" is 65% of 255 mm equalling 165.75 mm.

255 mm (10.04") is the value for the tire width.

Two sidewalls of 165.75 mm result in 331.5 mm and translate* to 13.05 " of rubber.

Add a 16 inch rim and you get a wheel with a 29.05" diameter.

So far so good -

and calculators offered at most tire web sites will agree with the result.


 BUT ...


The General Tire 255/65 R 16 has a published diameter of 28.21 inches.

That is almost one inch off. Why?/

Truth is that rarely ever two tires of the "same size" from different manufacturers will indeed have the same size.

Just check the 17" section and you see what I mean.

The only way to find out the exact size of a tire you intend to buy: Get the published value for "Static Load Radius" from the manufacturers and compare.

I hope to be able to publish all numbers here soon.

Send e-mail in case you can't get the numbers from your dealer - I have them in my database.


* 25.4 mm = 1.0 inch