Here is a description of various tire specifications:
1. Speed
2. Temperature
3. Traction
4. Speed
5. Load Index
6. Load Range
Speed Rating
How fast are you really going to go in your vehicle? You should never exceed the speed rating of a tire - nor should you be anywhere near the speed rating for a tire.
The speed rating of the tire is determined in a laboratory environment usually at a moderate ambient temperature. This does not factor in load due to cornering, nor heat due to hot asphalt or long periods of driving, nor inflation pressure that is not at the recommended level. Nor does a high rating rule out a defective tire. When you drive near or over the speed rating of a tire you are risking your life because there are always factors that cannot be taken into account. Car & Driver Editor Don Schroeder's life ended tragically as a result of high speed tire failure (second story on page).
As it turns out though, the higher the tire speed rating, the less likely you will have catastrophic sidewall or tread failure. It makes sense that higher speed-rated tires hold up better than lower speed-rated tires. This is not to say that you can overload a tire safely, or that you can safely have a blowout at 200 miles-per-hour - this just means that if a failure does occur at normal speeds, higher speed-rated tires generally handle the failure better.
The speed rating indicates the maximum speed (miles per hour) the tire is rated under normal conditions. The ratings are as follows:
M — 81
N — 87
P — 93
Q — 100
R — 106
S — 112
T — 118
H — 130
V — 149
W — 168
Y — 186
Below is a list of speed ratings along with the corresponding speeds they represent. Remember, the speeds are test speeds, not recommended speeds.
B - Up to 31 mph
C - Up to 37 mph
D - Up to 40 mph
E - Up to 43 mph
F - Up to 50 mph
G - Up to 56 mph
J - Up to 62 mph
K - Up to 68 mph
L - Up to 75 mph
M - Up to 81 mph
N - Up to 87 mph
P - Up to 94 mph
Q - Up to 100 mph
R - Up to 106 mph
S - Up to 112 mph
T - Up to 118 mph
U - Up to 124 mph
H - Up to 130 mph
V - Up to 149 mph
W - Up to 168 mph
Y - Up to 186 mph
Z - 149 mph and over
Speed ratings vary greatly depending on the type of tire. For example, Q-ratings are typical among off-road tires. R-ratings are typical for average passenger tires. T-ratings are typical for performance tires.
Provide for a 15-20% buffer and you should be OK. In other words, If you have a Q-rated tire, your sustained speed should not exceed 80-85 mph (unloaded) and less if you are carrying a significant load. (My Vanagon barely does 70, so a Q-rated tire is alright for my application.)
If you have a Porsche, and your maximum sustained speed is 135 mph, then you should not consider anything less than a W-rated tire. (Of course, this is merely an example and not advice to travel that fast.)
Again, the speed rating is determined at on an unloaded (unstressed) tire at a moderate ambient temperature. With that in mind, the next spec to take a look at is temperature.
Temperature Rating
Temperature and traction are probably the least marketed specifications for tires. However, in hotter climates, you should be especially concerned with temperature ratings.
The temperature rating indicates how well the tire handles heat (including how well the tire dissipates heat). There are three ratings:
A — best
B — intermediate
C — acceptable
Traveling long distances in hot weather, at a high-rate of speed, is very difficult on tires. Consider how many times you have seen a family on vacation in summer - with dad hauling down the highway with the wagon loaded down with the gear. This is a common situation that many times results in catastrophic tire failure.
Consider, also, how much more often you see semi-trailer tire tread on the highway during the summer. Luckily those big rigs have more than four tires.
Traction Rating
In wetter climates, the traction specifications may be more important than temperature.
The traction rating indicates how well a tire grips the road surface. This is an overall (balanced) rating for both dry and wet conditions. Tires are rated:
AA – best
A – better
B – good
C – acceptable
What is interesting is that many times, a tire's traction rating will be sold as how well the tire handles in wet weather. This isn't always the case - as the traction rating is a combined score. You could have an A-rated tire that performs excellent in dry weather, and acceptable in wet.
For most car owners, speed, temperature and traction are the most important specifications to consider.
Now we get to the important specifications for heavy (or top-heavy) vehicles - load index and load range.
Load Ratings
There are two load ratings: (1) Load Index, and (2) Load Range
The load index indicates the maximum weight that a tire can accommodate vertically at a given PSI (pounds per square inch). Typically, the load index of the tires for passenger cars and light trucks range from 80 to 100.
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
Sources: Tire Rack Tire Tech - Speed and Load Rating, and Yokohama - Reading Your Tire
Keep in mind that load ratings are determined using an applied vertical force against a tire under laboratory conditions (usually 65 ambient degrees). The heat rating for the tire can affect the maximum load the tire can handle at a given temperature.
You will see often the load index as a combination of Load and Speed, such as 97T. This indicates that the tire is rated for 1609 lbs. at a maximum of 118 mph. However consider that at that rate of speed, if you were to need to perform an emergency maneuver, you could easily exceed the load index of the tire.
Remember, the load index is how much weight you can put on top of your tires.
Load Range
Load Range is another way of indicating the ability of a tire to handle stress and weight. This time, we are indicating how much load can be placed on the tire horizontally. Often, these tires are associated with towing because the back end of the car tends to wallow while towing a trailer. In other words, you are exerting stress on the tire from side-to-side.
While there is no industry-wide definition of ply rating, truck tires are frequently marked with ply rating and equivalent Load Range. These markings are used to identify the load and inflation limits of that particular tire, when used in a specific type of service:
Load Range Ply Rating
A 2
B 4
C 6
D 8
E 10
F 12
G 14
H 16
J 18
K 20
L 22
M 24
N 26
Source: Bridgestone/Firestone North America, Sizes and Classifications
From me (stickshifter):
The Load Range for a mid-size SUV or light truck is usually a C-rated tire. But many off-roaders will move up to a D or even an E - not necessarily to get greater load-carrying capacity, but to get the thicker sidewalls that come on the D and E tires. A thicker sidewall will be more resistant to punctures from rocks, sticks, etc. There are disadvantages to the D and E tires, however. These include greater weight (so slower acceleration, longer braking distance, and worse fuel economy), and a rougher ride.
The Load Range for a heavy duty truck (Ford F-250, Ram 2500, etc.) is E or F (depending on the model, and its capacities). These tires can carry heavy payload, or tow heavy trailers due to their heavier construction.
Final note on load range: back in the day, a 10-ply tire had 10 plys, an 8-ply tire had 8 plys, etc. Since then, tire construction has changed, and that is no longer the case. So, while an E-rated tire can carry a heavier load than a D-rated tire, it does not necessarily have more plys, it is just a stronger construction. They keep the ply ratings alongside the letter-ratings as an artifact of a previous system of rating tire-strength. In other words, an E-rated tire is equivalent to what used to be a 10-ply tire.
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