Below is a chart from Les Schwab that visually breaks down the letters and numbers on your tires. For towing the main two you should know about is "F" load index and "G" speed rating.
Tire Size Meanings
A: TIRE TYPE The first letter in the code tells you what class of tire it is.
P stands for passenger vehicle tire. P-class tires include cars, SUVs, crossovers, minivans and smaller pickup trucks.
LT means light truck tire, designed for vehicles that are towing trailers or have ¾- and 1-ton load capacity.
ST stands for Special Trailer. These tire sizes are meant for trailers, including fifth wheels and other travel trailers, boat trailers and utility trailers.
If there's no letter before the first number, you have a metric tire most commonly referred to as European size. It's also measured in millimeters but may have different load capacity than a P or LT tire.
B: TIRE WIDTH The three-digit number following the letter is the tire's width (from side to side, looking at the tire head on) in millimeters. Also called the section width, this measurement is taken from outer sidewall to inner sidewall.
C: ASPECT RATIO The forward slash separates the tire width number from the two-digit aspect ratio. The bigger the aspect ratio, the higher/taller the tire's sidewall, or "profile" as it's sometimes called.
The aspect ratio is a percentage. It's the height of the sidewall measured from wheel rim to top of the tread, expressed as a percentage of tire width. In other words, it's sidewall height divided by tire width.
In this example, the aspect ratio is 65, meaning the sidewall is 65 percent as high as the tire is wide. To get the sidewall height, take the tire width of 215 mm and convert it to inches (8.46). Then multiply this by .65 and you get 5.5 inches, the sidewall height in inches.
D: CONSTRUCTION TYPE This single letter tells you about the internal construction of the tire.
R is for radial tires, the industry standard for most tires today. They have better road grip, lower rolling resistance for better gas mileage, ride comfort and durability than previous generations of tires. In a radial tire, the plies - layers of strong cords made of a blend of polyester, steel and fabric and coated with rubber - are laid perpendicular to the direction of travel.
E: WHEEL DIAMETER This two-digit number specifies wheel diameter in inches, how wide the wheel is across the center. It's the distance between the two bead seat areas (where a tire gets slotted and tightly sealed onto the wheel).
F: LOAD INDEX The two-digit or three-digit number that follows the gap specifies tire load index. The load index symbol indicates how much weight a tire can support, based on the following standard chart. In our example, the load index is 89, which indicates the tire has a load capacity of 1,279 pounds.
G: SPEED RATING The last letter is the speed rating, which tells you the top speed it's safe to travel at for a sustained amount of time. A tire with a higher speed rating can handle heat better and provide more control at faster speeds. The maximum operating speed of a vehicle is no more than the lowest speed rating of all tires mounted on the vehicle. (Of course, you should always abide by speed limits for safer driving.) Speed rating is usually, but not always, a single letter (see the chart).
To read Les Schwabs full article click here: https://www.lesschwab.com/article/tire-size-explained-reading-the-sidewall.html