Have you ever taken a good look at the tires on your truck and noticed a bunch of numbers and letters on the sidewall? What do they represent? We have the low down on what this means, so the next time you stop into the tire store, you can know what you're talking about.
A typical tire size will look something like this LT 275/70R18 122S
The very first letter or letters in the sequence is the tire type. On most heavy-duty trucks, you will see LT (LT-metric) which means a light truck type tire. These types of tires are used on vehicles that pull trailers or carry heavy cargo. You might also see AT for all-terrain or MT for mud terrain. Another common one is P, which stands for passenger and is on some smaller pickup trucks.
The next number is the measurement of the width of the tire from sidewall to sidewall. This is displayed in millimeters. For comparison purposes, 25.4 millimeters = 1 inch.
After the first slash mark, the next number is the aspect ratio of the tire. This ratio is determined by dividing the tire's section height or side wall height with the width of the tire. This number is given as a percentage. A tire with lower aspect ratio has a shorter sidewall or lower profile which allows the tire to handle lateral forces better. This also means a lower aspect ratio will have a more responsive feel when driving.
The next letter describes the tire's composition. There are two letters you will see:
R – Radial, which means the tire’s plies run at 90 degrees to the centerline of the tread
All modern vehicles use Radial tires as a standard. They allow a smoother ride and stiffer sidewall that keeps the vehicle more stable on the road.
D – Diagonal or Bias Ply, which means the plies run at angles lower than 90 degrees
If you are restoring a pre 1970s truck it most likely had Bias Ply tires on it. If you are looking to make it all original you may want to look for a Bias Ply tire, but be prepared to sacrifice the modern handling if you choose this route.
DIAMETER OF RIM
Next, this number tells the diameter of the wheel that the tire can be mounted on. This is measured in inches.
The load index tells you how much weight the tire can support when properly inflated. The higher the tire's load index number, the greater its load carrying capacity . LT Tires have two load index ratings because these type of tires are used on vehicles for dual rear wheels.
The last letter is the speed rating. Every tire that is approved by the DOT (Department of Transportation) is given a speed rating. Here are some common ratings for trucks:
L - maximum speed is 75 mph
Q - maximum speed is 99 mph
R - maximum speed is 106 mph
S - maximum speed is 112 mph
T - maximum speed is 118 mph
U - maximum speed is 124 mph
People often refer to their tire sizes as 34 inch or 35 inch tires. This is not a standard tire sizing and you need to use a converter to calculate the metric tire sizes to figure out what sizes will get you this inch size. It is a combination of the tire width, aspect ratio, and wheel diameter.