How to Read your Tire's Sizing Code - The Complete Guide!

How to Read Tire Sizing

Are you ever left confused and speechless when you are confronted with the question of "what are is your tire size?" It sure is confusing with all these numbers and letters in a weird foreign code of which you have not been taught in. But don't worry, our tire sizing guide will decode this foreign language for you! 


Source: google

Look for these numbers on your tires to determine the specifications of the tires.

Tire type

The P at the beginning tells us the tire type, referring to the manufacturing standards of the tire, made for specific classes of vehicle.
P = Passenger Vehicles (ie cars, minivans, light-duty pickup trucks and SUVs)
LT (code begins with LT) = Light truck (For vehicles used to tow trailers or carry heavy loads; SUVs, full-size vans, medium- and heavy-duty pickup trucks)
LT (code ends with LT) = Refers to the tire as either a "Numeric", "Wide Base" or "Flotation" Light truck size (For vehicles capable of carrying heavy cargo and towing trailers)
ST = Special Trailer (To be used on car, boat or utility trailers)
T = Temporary (Temporary spare tires designed for short-term use until the regular tire is repaired/replaced)
C = Commercial (For use on delivery trucks and vans capable of carrying heavy loads)

If there are no letters in the front of the numbers, don't worry because it only means that they are Metric (also known as Euro-Metric) tires. These tire sizing originated in Europe and are equivalent to the P-Metric (P-metric, as shown above, are the most common tires) sizes in the dimensions. Metric tire sizes have very subtle differences in the load-carrying ratings and capacities. The Metric sizes ending with a C refer to the Commercial tires. 

Section Width

The 3 numbers after the initial letter (if applicable) refer to the tire's section width. It is the cross section measurement of the tire in millimeters.
For the above example, it is a P185/75R14 which means it is a passenger tire and the width of the tire is 185mm wide.
This measurement is measured from the widest point of the outer sidewall to the widest point of the inner sidewall of the tire when properly mounted. 

Aspect Ratio

The 2 numbers after the section width refer to the tire's profile/aspect ratio. It is a ratio determined by the height and width of the tire. 
In the above example P185/75R14, the aspect ratio 75 indicates that this specific tire size's sidewall height (from the wheel rim to the tread) is 75% of its section width (recall; section width is the cross section measurement/width of the tire).
So what does this mean anyways? Basically, the higher the number for the aspect ratio is, the taller the sidewall and the lower the number, the lower the sidewall. 

Internal Construction

The single letter after the 2-digit aspect ratio will tell you about the tire's construction. 
In the above example P185/75R14, the R stands for radial. Most tires today have radial construction. This means that the internal body plies of the tire radiate outward from the center. 
There is also D, which refers to a bias ply construction, where the internal body plies of the tire crisscross on a diagonal pattern. Tires using this construction are for light truck and spare tires.
If there is a B, referring to a belted tire, the internal plies crisscross in the same manner as a D construction, but there's an extra layer or reinforcing belts under the tread area. 

Tire and Wheel Diameter

The 2-digit number following the internal construction letter refers to the diameter of the wheel to be matched to this certain tire. 
In the above example P185/75R14, a 14-inch wheel needs to be used with the tire for proper fitment. Sometimes this number may be displayed in millimeters, simply convert the number by dividing by 25.4 to obtain the sizing in inches. There may also be rim diameters expressed in half inches - mostly for heavy-duty trailers, heavy-duty light trucks and box vans.

Load and Speed Rating

The 2-digit number followed by a single letter refers to the load and speed ratings respectively. Collectively it is also referred to as the Service description
In the above example, P185/75R14 82S, 82 refers to the load rating and S is the speed rating. The load rating is used to to tell you how many pounds the tire can safely carry. Now let me reassure you that the number for the rating shown does not correspond to the amount of weight it can carry (ie 82 does not equal 82 pounds or 82 kilos). There is a handy reference list.
It is very important to maintain a proper load index for your vehicle when you replace your tires.
The speed rating ensures the tire will match the vehicle's top speed capability. It is necessary to use a tire that will match your vehicle's top speed capability. Do not use tires with different speed ratings as it will affect your handling. Generally tires with higher speed ratings will help get rid of heat better as they have higher quality construction and ply. This will allow for better cornering, gripping and braking. 

 Typically passenger vehicles have a load index ranging from 70 to 110.

Load Rating Chart

Load Index Pounds Kilograms  Load Index Pounds Kilograms
71 761 345 91 1356 615
72 783 355 92 1389 630
73 805 365 93 1433 650
74 827 375 94 1477 670
75 853 387 95 1521 690
76 882 400 96 1565 710
77 908 412 97 1609 730
78 937 425 98 1653 750
79 963 437 99 1709 775
80 992 450 100 1764 800
81 1019 462 101 1819 825
82 1047 475 102 1874 850
83 1074 487 103 1929 875
84 1102 500 104 1984 900
85 1135 515 105 2039 925
86 1168 530 106 2094 950
87 1201 545 107 2149 975
88 1235 560 108 2205 1000
89 1279 580 109 2271 1030
90 1323 600 110 2337 1060

 

Speed Rating Chart

 Rating Maximum Speed (mph) Maximum Speed (km/h) Type/Vehicle Type
L 75 mph 120 km/h Off-Road & Light Truck
M 81 mph 130 km/h Temporary Spare
N 87 mph 140 km/h Temporary Spare
Q 99 mph 160 km/h Winter 4x4
R 106 mph 170 km/h Heavy Duty Light Truck
S 112 mph 180 km/h Family Sedans & Vans
T 118 mph 190 km/h Family Sedans & Vans
U 124 mph 200 km/h Sedans & Coupes
H 130 mph 210 km/h Sport Sedans & Coupes
V 149 mph 240 km/h Sports Car
Z 149+ mph 240+ km/h Sports Car
W 168 mph 270 km/h Exotic Sports Cars
Y 186 mph 300 km/h Exotic Sports Cars
(Y) 186+ mph 300+ km/h Exotic Sports Cars

 
And there you have it - now you know how to decode tire specifications! 

There are also other important information you can gain from reading the side wall of the tire, such as the date the tire was manufactured, and if you want to learn how to decode that information - stay tuned! 

The next time someone asks you for your tire size, just remember the code goes by: Type, Width, Ratio, Construction, Rim Diameter and then Load and Speed Rating. 

Source: google