Anyone know why my 315/70r17 BFG ATs measure 33.5"? I know you can have different height on the same tire for different width wheels, but not sure what the actual width change does to height as far as a number goes. Like, going from my stock 7.5" wide wheels to 9" wide wheels, will that give me more or less height? I know it happens because the sidewalls are "puckered" in and that brings some of the edge tread toward the center of the circle. But Heightwise, how does it work?

 

go to tirerack.com and find the tire calculator. I thinkit is on the top right of their page.

 

A wider wheel will "widen" the tire up causing it to "shrink" in height, just as a narrower wheel will make it "grow".

 

 

First number is tire width in millimeters from tread cap to tread cap, second number is the side wall height in a percentage relationship to the tread width (your side wall height is 70 % of your tread width of the size you posted) the last number is the rim size in inches.

 

The second number isn't a relationship to the tread. It is a percentage of the section width, which is wider than the tread. The section width includes the distance from each side of the outside of the sidewall. The sidewall height is a percentage of this measurement.

 

315/25.4 X 70% x 2 + 17 = 34.36" However diff mfg tires measure diff. My BFG AT's 285/70/17 on 8" rims measure 32" not 32.7". But Interco MT 33x12.50 are actually 33" tall on the Jeep.

 

No no no. I know how to calculate tire height from the mm measurements. I know all that. What I wanted to know was how the rim WIDTH affects height. Redneckjeep answered it. "A wider wheel will "widen" the tire up causing it to "shrink" in height, just as a narrower wheel will make it "grow". " If that was the case though, my 315/70r17 (34.4") on my skinny stock wheels should be taller than 34.4", but in fact they are 33.5" when measured.
I thought I read once that skinny wheels do make the tire taller if the same volume of air is used, but in order to get all the tread touching the ground, air needed to be let out to push the sides back down to the pavement and lower pressure back to normal. I think it has to do with volume of air in the tire and pressure and having the tread all touching the ground. I'll just look it up. Gotta be something on it somewhere.

 

First, I assume you are measuring the tire height with no load right?

but actually, tire dimensions does not appear to be a highly regulated thing. I remember seeing a Peterson's 4WD article years ago comparing several "35-inch" MT's installed on identical rims & rigs for test purposes. The tire heights varied y as much as 2 or 3 inches (with one tire being measured a 36, while another was a 33 or so).

 

Correct - generally, the wider rim yields a shorter mounted tire, and the narrower rim yields a slightly taller tire....its a minor difference.

The larger difference, as mentioned, is that the sizes are NOMINAL...so while the MATH might say a tire will be 33.47554404004" n diameter and 315 mm is section width....it might commonly be anywhere from 32" - 34" in diameter....etc.

The REALLY big tires vary even more...like a "44" might be 38", etc.