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.
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 me an email in case you can't get the numbers from your dealer  I have them in my database.
* 25.4 mm = 1.0 inch
