Products Showing Parts for — {{ active_vehicle.name }}

No Results Showing Parts for — {{ active_vehicle.name }}

Cummins, Duramax, Installations, Powerstroke

Truck Tires 101

Posted on by

0

Truck Tires 101

There’s a quite a bit of information on the sidewall of every tire, but it’s written in an alpha-numeric code and can be hard to understand. The size of the tire is just one small piece of this code. The tire size P325/60R20 or 35×12.50R20LT may not mean much to the average person, but it can be extremely important when purchasing the correct set of tires for your truck. In Truck Tires 101, we discuss how to decipher the codes on a tire.

How to decipher inch sizes: 

35x12.50R20LT 125Q 12PR

35×12.50R20LT 125Q 12PR

Diameter : 35
The first number is the overall diameter of the tire. The diameter of this tire is 35 inches.

Section Width : 12.50
The second number is the width of the tire in inches. The width of this tire is 12.5 inches.

Tire Construction : R
The tire construction of this tire size is ‘R’ for radial. This letter describes how the tire is constructed.

Wheel Diameter : 20
The wheel diameter of this tire size is 20 inches. The tire is designed to be mounted on a 20 inch wheel.

Tire Class : LT
The tire class of this tire size is ‘LT’ which means it is a Light Truck tire.

Load Index : 125
This number is the load index and the maximum amount of weight the tire can carry. A higher number means the tire can carry more weight. The load index of this tire is ‘125’ – 3,6384 lbs.

Speed Rating : Q
The speed rating of this tire size is ‘Q’, or 99 mph. This letter is the maximum speed rating of the tire.

Load Range : 12
Tire load range of this tire size is 12, which means it has a 12 ply tread. This can be a letter or a number, other load ranges include B: 4 ply, C: 6 ply, D: 8 ply, E:10 ply, and F:12 ply.

See tables below for Load Index, LT Load Ranges, Speed Ratings and Tire Construction.

How to decipher metric sizes:

P325/60R20 10PR 126/123R

P325/60R20 10PR 126/123R

Tire Class : P
The tire class of this tire is ‘P’ which means it is a passenger car tire. The first letter in the tire size designates the tire class. An ‘LT’ would be a light truck class and no letter would be a euro-metric tire. Note: ‘P’ Tires includes cars, minivans, sport utility vehicles and light duty pickup trucks (typically 1/4- and 1/2-ton load capacity).

Section Width : 325
The first number in a metric tire size is the section width of the tire. The width of this tire size is 325mm.

Aspect Ratio : 60
The second number in a metric tire size is called the aspect ratio. The aspect ratio on this tire size is 60, which means that the sidewall height is 60% of the section width.

Tire Construction : R
The tire construction of this tire size is ‘R’ for radial. This letter describes how the tire is constructed.

Wheel Diameter : 20
The wheel diameter of this tire size is 20 inches. The tire is designed to be mounted on an 20″ wheel.

Load Range : 10
Tire load range of this tire size is 10 which means it has a 10 ply tread. This can be a letter or a number, other load ranges include B: 4 ply, C: 6 ply, D: 8 ply, E:10 ply, and F:12 ply.

Load Index : 126/123
This number is the load index and the maximum amount of weight the tire can carry. A higher number means the tire can carry more weight. Since LT tires are commonly used on trucks with dual rear wheels, they are branded with two load indexes. The first number indicates the load carrying capacity if the tire is installed on a truck with a single-wheel rear axle, and the second number applies when the tire is used in a dual rear application. The load index of this tire is “126” – 3,748 lbs for a single wheel rear axle, and “123”  – 3,417 lbs for a dual rear wheel application.

Speed Rating : R
The speed rating of this tire size is V – 106 mph. This letter is the maximum speed rating of the tire.

See tables below for Load Index, LT Load Ranges, Speed Ratings and Tire Construction.

Converting Metric to Inches

Now we will convert a metric tire measurement into inches, so it might help to have a calculator handy.

Let’s use the same metric tire from above as an example: 325/60R20

On metric tires the first number is the Section Width of the tire in millimeters. In this example it reads 325mm. To convert 325mm to inches you simply divide 325 by 25.4, since there are 25.4 mm in one inch. So the width of a 325mm tire is 12.80 inches. The next number is typically referred to as the Aspect Ratio or the proportional relationship between the width and height. This tells you the sidewall height of the tire. For our example the sidewall height of the tire would be 60% of 12.80″ which is 7.68″. Finally, to calculate the overall diameter of the tire you simply need to add the sidewall measurement twice (for the top and bottome sidewall), and the diameter of the wheel.

If we add the sidewall twice, it is 7.68 + 7.68 = 15.36. Next add 20″ for the wheel diameter, so the tire height 15.36 + 20 = 35.36. According to these calculations a 325/60R20 metric tire would result in an inch measurement equivalent of a 35.36X12.80R20 tire. As you can see this metric size tire is basically the equivalent of a 35×12.50R20. Not all tire sizes are manufactured in both metric and inch sizes, so learning how to read a tire size can be a valuable tool for anyone in the market for new tires.

Tire calculators can be found online, and there are free apps available which will do the math for you. Important information such as the height, load rating, speed rating, etc., can also be found directly on the tire manufacturers website.

Check back for our next installment – Truck Tires 102. In part 2, we begin to breakdown all of the tire terminology and what the ratings mean.

Load Index

Load
Index
Max lbs. Load
Index
Max lbs. Load
Index
Max lbs.
71 761 92 1389 112 2469
72 783 93 1433 113 2535
73 805 94 1477 113 2535
74 827 95 1521 114 2601
75 853 96 1565 115 2679
76 882 97 1609 116 2756
77 908 98 1653 117 2833
78 937 99 1709 118 2910
79 963 100 1764 119 2998
80 992 101 1819 120 3086
81 1019 102 1874 121 3197
82 1047 103 1929 122 3307
83 1074 104 1984 123 3417
84 1102 105 2039 124 3527
85 1135 106 2094 125 3638
86 1168 107 2149 126 3748
87 1201 108 2205 127 3858
89 1279 109 2271 128 3968
90 1323 110 2337 129 4079
91 1356 111 2403 130 4189

LT Load Ranges

Load Range Ply Rating Max Load Pressure
B 4 35 psi
C 6 50 psi
D 8 65 psi
E 10 80 psi
F 12 95 psi

Speed Ratings

Letter Max
Speed
L 75 mph / 120 km/h
M 81 mph / 130 km/h
N 87 mph / 140km/h
P 93 mph / 150 km/h
Q 99 mph / 160 km/h
R 106 mph / 170 km/h
S 112 mph / 180 km/h
T 118 mph / 190 km/h
U 124 mph / 200 km/h
H 130 mph / 210 km/h
V 149 mph / 240 km/h
W 168 mph / 270 km/h
Y 186 mph / 300 km/h

Tire Construction

Letter Construction Type
B Bias
D Diagonal
R Radial