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"
So far so good -
and calculators offered at most tire web sites will agree
with the result.
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
in case you can't get the numbers from your dealer - I have them in
* 25.4 mm = 1.0 inch