tire size - 8-17.5LT?

I am working on a 58 GMC Model 150, I need the tire size so I can figure out what size of ring and pinion gears i need, the tires say 8-17.5LT, does that mean sidewall is 8 in, rim size is 17.5 meaning diameter is 31.5?

An 8R17.5 is 31.8" in diameter. Don't forget to deduce 3% to account for the difference between the physical diameter and the rolling diameter.

what i the physical diameter and the roling diameter? nut i would take that off of 31.8?

The physical diameter is what you can actually measure. In this case 31.8".

The rolling diameter is the diameter the tire appears to be rolling, which can't be measured directly - only by counting revolutions over a given distance. In this case we can estimate the rolling diameter = 31.8*0.97 = 30.85"

I dont think it will mater too much, i tried 30 in and 31 in and 31.8 in in the equation and they were all so close it wont matter
thanks