For those who don't know how to read and work with tire sizes:
LT stands for Light Truck and P stands for Passenger. If a tire doesn't list LT or P, I assume it's a passenger vehicle tire.
I'll sometimes use 'P' tires (or tires without a 'use' designation) on 1/2-ton trucks, depending on how I plan to use it primarily. On 3/4-ton trucks I prefer 'LT' tires. On 1-ton and heavier, I only use 'LT' tires.
The first number is the tread width, in millimeters. I use my computer's calculator to convert millimeters into inches because it's proven to be accurate. (When I get a new computer or operating system software, checking the accuracy of the provided calculator is one of the first things I do ... using a variety of conversions and calculations.)
In the recent discussion, we've been using 235 millimeters for thread width ... this converts to 9.25 inches. (Actually, 9.251968503947 inches but we don't need that decree of accuracy ... as long as we're not using furloughs per fortnight.)
The second is the sidewall height, as a percentage of tread width. if the tire size is 235/75, the sidewall height is 6.9375 inches. (9.25 inches times 75 percent -- or 0.75) For 235/85, sidewall height is 7.8625 inches. (9.25 inches times 85 percent -- or 0.85)
(R stands for Radial ... bias ply tires are rare now so the R is kept mainly because that's the way it's been done since the switch to metric tire sizes.)
Third number is rim diameter, in inches.
Tire diameter is sidewall height time two (both sides of rim) plus rim diameter.
So, LT235/75R16 tires are 29.875 inches in diameter. (9.25 times 0.75 times two plus 16) LT235/85R16 tires are 31.725 inches in diameter. (9.25 times 0.85 times two plus 16)
Tire circumference is tire diameter time pi. (3.14159 if you like doing things the long, tedious -- but more reliable -- way I prefer.)
