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Cummins, Duramax, Powerstroke, Tech Talk

Tires 102

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In our first installment ‘Tires 101‘, we dipped one toe into the water and started going over the basics of tires. We also learned how to decipher metric tire sizes, and convert metric sizes to inches. Now we will go into a little more detail about tire construction and further define some of the terminology written on the side of the tire.

Tire Class

These letters before (or after) the size convey two things, ply rating, which speaks to load-carrying capability, and tire type. No letter or a P, represents a standard load (4 ply rating) “passenger” tire, or an extra load tire. Extra load tires offer more weight capacity than a normal standard load, but not as much as an LT tire. If your tire is extra load, it will have an XL after the size such as P255/75R20XL.

LT and ST tires are all heavier weight capacity tires. They can be Load Range C (6 ply rating), D (8 ply rating), or E (10 ply rating). They can even be F or G (12 and 14 ply) on motor homes or trailers. LT means “Light Truck” and is for pickups and vans. ST refers to “Special Trailer” and are “for trailer service only”.

Half-ton or smaller pickups and SUVs often use P metric, standard load tires. 3/4 and 1-ton pickups & vans must use higher load ranges, usually D or E. In other words, you can go higher in ply rating on half tons and SUVs, but you never want to go lower than D on 3/4 and 1 tons.

ST tires are not designed for the requirements of a motor vehicle, and should never be used on anything but a trailer. Standard load tires can be used on trailers, but only very light ones. Heavy trailers should always use higher ply rating tires (ST or LT) that have sufficient weight capacity for the trailer when fully loaded.

Tire Construction

The letter before the rim diameter represents the “construction type” of the tire. There are two types, Bias and Radial. Bias tires have belts in the tire that have a “Bias” or slant, forming an X pattern across the tire. ‘B’ and ‘D’ are both Bias belted tires – ‘B’ stands for “Bias”, and ‘D’ stands for “Diagonal”.

Radial tires, which have an ‘R’, have belts that go “Radially” straight across the tire (or perpendicular to the tread). The vast majority of tires sold nowadays are radial and for good reason. When a bias tire’s tread hits the ground, the diagonal belting causes the tread to compress together. A radial’s tread, on the other hand, stays flatter and instead flexes in the sidewall. The folding action of the bias tire inhibits traction, particularly on slippery surfaces, and also causes drag and heat build-up. This results in the radial tire offering better traction, better fuel mileage, and substantially better wear.

You might wonder why they still sell bias tires at all. Apart from cost, there is one good reason: Bias belted tires seem to stand up better to age. So if you have an old truck or trailer that rarely gets used, a bias tire might actually be a better choice.

What Is Load Range, Ply Rating, Load Index & Speed Rating?

Often, you will see a number and letter following the tire size. This set of information denotes the load and speed rating. On some tires, you may see three letters instead of one. The first gives the speed rating while the next two indicate the load range.

Load Range and Ply Rating

The load range is also known as ply rating — a term that dates back to the days of bias-ply tires, which had different numbers of ply layers. More layers indicated a better and stronger tire. Today, ply rating still refers to the strength of the tire, but since fewer, stronger plies are used, the numbers have been replaced in some cases by load range letters.

The load ranges for passenger tires have a listing beneath the tire code with information about the maximum load range and tire pressures. The load range indicates the heaviest load the tire can handle in pounds. Maximum tire pressure indicates the highest you can inflate the tires, but do not fill your tires to this pressure. It reflects the highest pressure the tire can handle when carrying its maximum load. Use the pressure given in your vehicle’s owner’s manual for the ideal tire pressure that creates the most comfortable and safest ride.

Light truckload ranges have letters B, C, D, E, and F, and they have increasing maximum pressures. The lowest pressure is for B tires with 35 psi, and C has a pressure of 50 psi. The pressures increase in increments of 15 from 65 psi, 80 psi, and 95 psi for tires with load ranges of D, E, and F. For work trucks, the most common load ranges are C, D, and E. Commercial trucks, which have much higher carrying requirements, have load ranges from F through L.

In some cases, you may see a number after the load range number, for example, C1 or C2. They still have the same number of plies, but the numbers designate differing max pressures.

LT Load Ranges

Load RangePly RatingMax Load Pressure
B435 psi
C650 psi
D865 psi
E1080 psi
F1295 psi

Load Index

The load index on a tire gives a numerical value for the maximum weight the tire can handle when it is inflated to its recommended pressure. Higher numbers correlate to the tire’s ability to bear heavier loads. Indices start at 1, which can carry 102 pounds, and they go up to 150, which can hold up 7,385 pounds. In the example, 35×12.50R20LT 125Q 12PR, the 125 load index means the tire can carry up to 3,638 pounds.

Like tire load ratings, the higher the load index, the more robust the tire is. Higher load indices correspond to tires used for hauling cargo or for installation on work trucks. No matter how strong the tire is, though, it’s important to remember to never carry weights that are heavier than your tires or vehicle’s suspension are rated for.

If you see a pair of load ratings, this is not a mistake. Instead, these LT tires have an additional load rating listed for dual tire application. The first number gives the load index for the tires when used on a single axle. The second number is how much the tire can handle when used on a dual axle.

For instance, if you see a 126/123 for the load index, this means the tire has an index of 126 when used by itself but only 123 when installed on a dual-tire axle. The second number is usually lower because of unequal load sharing when the tires are paired together. However, when the tire is acting by itself, it can support a higher weight.

Load Index

Max lbs.Load
Max lbs.Load
Max lbs.

Speed Rating

Typically, for trucks, speed is not the most critical factor, which explains why a speed rating of R for heavy-duty trucks has a limit of only 106 mph. Those who drive sports cars or race usually require tires built for higher speeds. If you need to haul cargo, look for the load index or range, which will help you to identify the best tires for your operation.

In the example tire, P325/60R20 10PR 126/123R, the R indicates the speed index of up to 106 mph.

Speed Ratings

L75 mph / 120 km/h
M81 mph / 130 km/h
N87 mph / 140km/h
P93 mph / 150 km/h
Q99 mph / 160 km/h
R106 mph / 170 km/h
S112 mph / 180 km/h
T118 mph / 190 km/h
U124 mph / 200 km/h
H130 mph / 210 km/h
V149 mph / 240 km/h
W168 mph / 270 km/h
Y186 mph / 300 km/h

Choosing the Correct Tire

Hopefully, some of this information will be helpful next time you purchase tires. As a baseline, it is a good idea to choose tire sizes and the minimum load ratings recommended by the manufacturer. The demands of your tires can change, depending on how you use your truck. If you pull a trailer or haul heavy loads on a regular basis, don’t forget to take that into consideration when shopping for your next set of tires.