The tire load range designates how much horizontal stress a tire can take. This is particularly important for light truck tires that are used in towing. The letter used to designate load rating converts to a measurement of pounds per square inch (psi). The load range no longer represents the total number of plies in the tire. Instead, it represents the strength of the tire relative to the strength of plies used at the time load ranges were introduced.
Determine whether you have a passenger tire or a light truck tire by looking at the tire identification number on the side of your tire. The passenger tire numbers start with "P," while those on light trucks begin with "LT." If you have a light truck tire, skip to Step 3.
Locate the load range indicator on your passenger tire. Passenger tires will be marked with "LL," "XL" or no indication at all at the end of the tire identification number on the sidewall. If there is no indication, the tire is a standard-load tire that can be inflated to 35 psi. If you have "LL," the tire is a light-load tire that can also be inflated to 35 psi but cannot take as much horizontal strain. If you have "XL," you can inflate the tire to 41 psi.
Locate the load range letter on your light truck tire. The load range indicator letter sits by itself in the middle of the tire identification code. The load range indicator letter generally is between "B" and "F" but can be as high as "N."
Convert the letter to ply rating by multiplying the number the letter is in the alphabet by two. For example, a "D"-rated tire would be eight-ply because D is the fourth letter of the alphabet and four times two is eight.
Convert the ply rating to psi using a load-range conversion chart. For example, a light truck tire with a rating of "D" would be inflated to 65 psi.
