You guys are both very close!
So, here’s an easy way to figure out overall tire diameter:
The first number, in this case, is 315. 315 is referring to the actual WIDTH of the tire, measured in millimeters. Many think it’s referencing diameter, but it’s not.
The second number, 70, is actually an aspect ratio, in percentages, of the width of the tire. The 70 aspect ratio is the SIDEWALL HEIGHT of the tire. Now remember, there’s two sidewall heights, the height between your wheel and the ground, and the height from the top of the wheel to top of tire.
To convert to inches, divide the 315 by 25.4(25.4mm’s to an inch), and you get 12.4”. That’s the width of the tire.
Now the sidewall height is the 70 aspect ratio so take 12.4 x .70 and you get 8.68”. Remember, two sidewall heights so multiply 8.68 x 2 and you get 17.36”.
Now all you need to do to get TOTAL overall diameter is add in the wheel diameter, 17 in this case, and you end up with 34.36”!
Kind of easy? Lol I learned that years ago with performance cars when you were always trying to go with bigger wheels and lower profile tires, but still keep overall diameter the same. It’s definitely very helpful!