Understanding
Tire Markings and Ratings
"Tire
Type"
defines the proper use of the tire. P means this is a passenger car tire. An LT
in the Tire Size means that the tire fits a light truck.
"Tire
Width" is the width of the
tire measured in millimeters from sidewall to sidewall. The example shown above
is 215 millimeters.
"Aspect
Ratio" is the the ratio of
the height of the tire's sidewall to its width. 65 means that the height is
equal to 65% of the tire's width.
"Construction" tells you how the
tire was put together. The "R" stands for radial, which means that the body ply
cords, which are layers of fabric that make up the body of the tire, run
radially across the tire from bead to bead. A "B" indicates the tire is of bias
construction, meaning that the body ply cords run diagonally across the tire
from bead to bead, with the ply layers alternating in direction to reinforce one
another.
"Wheel
Diameter" is the width of the
wheel from one end to the other. The diameter of the wheel in the example above
is 15 inches.
"Service Description" is the combination of the Load Index and Speed Rating described below. For example, the Service Description shown in the tire diagram above is "89H". This tire has a Load Index of 89 (rated for 1289 pounds or 580 kg) and a Speed Rating of "H" (up to 130 mph).
"Load
Index" is a number
corresponds to the maximum load in pounds that a tire can support when properly
inflated. You will also find the maximum load in pounds and in kilograms molded
elsewhere on the tire sidewall.
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 |
"Load
Range" In
addition to the “Load Index” the “Load Range” of a tire gives the strength and
ability to hold air pressure, based on the sidewall ply of the tire, and is
denoted
by letters (B, C, D, E, etc.).The term
“Load Range” replaces the term “Ply Rating” formerly used to rate tire strength.
Most passenger car tires are rated Standard (STD) or Standard Load (SL).
Standard Load tires will not have their load range branded on the sidewall.
Extra Load or Reinforced tires will have “XL” or “RF” branded after the tire
size.
Load
Range and Load Pressure for Passenger Tires
|
Load
Range |
Load
Pressure (psi) |
P-metric |
Light
Load |
35 |
Euro-metric |
Standard |
36 |
Since
light truck tires are often available in multiple load ranges, the appropriate
load range is identified immediately following the size's rim diameter in The
Tire Rack's descriptions.
Load
Range, Ply Rating and Load Pressure for Light Truck Tires
|
Load
Range |
Ply
Rating |
Load
Pressure (psi) |
LT-metric |
B |
4 |
35 |
LT-numeric |
C |
6 |
50 |
Flotation
LT* |
D |
8 |
65 |
*Selected
large Flotation LT sized tires have reduced load pressures from the values
shown above. |
ST-metric |
B |
4 |
35 |
"Speed
Rating" is a number that
corresponds to the maximum service speed for a tire. "H" means that the tire has
a maximum service speed of 130 mph. Please note that this rating relates only to
tire speed capability, and is NOT a recommendation to exceed legally posted
speed limits; always drive within the legal speed limits. Speed ratings are
certified maximum sustained speed designations assigned to passenger car radials
and high performance tires. Because of the evolution of high-speed passenger car
travel, it was necessary to establish a way to rate a tire's high-speed
capability. In the U.S., these ratings are based on tire testing in laboratory
conditions under simulated loads (European testing uses actual road testing).
For a tire to be speed rated by the U.S. Government, it must meet certain
minimum government standards for reaching and sustaining that specified speed.
Domestically, high performance tires must be speed rated. The tire industry
defines high performance tires as those with speed symbols of "S" or greater and
aspect ratios of 70 or lower. Conventional passenger car radials need only meet
the minimum Department of Transportation standard of 85 mph. Speed symbols may
currently be marked on a tire in any of three ways: 205/60ZR15; 205/60ZR15 89W;
or 205/60R15 89W.
Speed
Symbols | ||||||
Speed |
Speed |
Speed |
|
Speed |
Speed |
Speed |
M |
130 |
81 |
|
U |
200 |
124 |
N |
140 |
87 |
|
H |
210 |
130 |
P |
150 |
94 |
|
V |
240 |
149 |
Q |
160 |
100 |
|
W |
270 |
168 |
R |
170 |
106 |
|
Y |
300 |
186 |
S |
180 |
112 |
|
Z |
above
300 |
above
186 |
T |
190 |
118 |
|
|
|
|
"DOT" means the tire is
compliant with all applicable safety standards established by the U.S.
Department of Transportation (DOT). Adjacent to this is a tire indentification
or serial number; a combination of numbers and letters with up to 12
digits.
"UTQG" stands for Uniform
Tire Quality Grading, a quality rating system developed by the Department of
Transportation (DOT) that measures each tire for quality of treadwear, traction,
and temperature.
Except
for snow tires, the DOT requires the manufacturers to grade passenger car tires
based on three performance factors: tread wear, traction, and temperature
resistance. The UTQG rating of each Goodyear passenger car and light truck tire
is listed in the Car/Light Truck Tire Catalog.
Tread
Wear
The
tread wear grade is a comparative rating based on the wear rate of the tire when
tested under controlled conditions on a specified government test track. A tire
graded 200 would wear twice as long on the government test track as one graded
100. Your actual tire mileage depends upon the conditions of their use and may
vary due to driving habits, service practices, differences in road
characteristics and climate. Note: Tread wear grades are valid only for
comparisons within a manufacturer's product line . They are not valid for
comparisons between manufacturers.
Traction
Traction
grades represent the tire's ability to stop on wet pavement as measured under
controlled conditions on specified government test surfaces of asphalt and
concrete. The Traction grade is based upon "straight ahead" braking tests; it
does not indicate cornering ability.
Temperature
The
temperature grades represent the tire's resistance to the generation of heat
when tested under controlled conditions on a specified indoor laboratory test
wheel. Sustained high temperatures can cause the materials of the tire to
degenerate and thus reduce tire life. Excessive temperatures can lead to tire
failure. Federal law requires that all tires meet at least the minimal
requirements of Grade C.
Directional
Directional
tires can only spin in one direction. The direction of spin should be indicated
by an arrow on the side of the tire.
Asymetrical
Asymetrical
tires are designed to only be used front or rear, but not both. The tires will
conform to the different properties of each axle for better
performance.
Run-Flat Tires
Run-flat tires have been available as factory equipment on several high-end cars for a number of years, including the Chevrolet Corvette. These tires feature a special design that keeps them locked in place on the wheel and maintains drivability. Some run-flat designs simply add enough sidewall stiffness to low-profile tires to operate at conservative speeds without air. Newer designs incorporate a composite liner around the wheel inside the tire to add support if air pressure is lost. That means you can travel with zero air pressure for a considerable distance (as far as 50 miles at 55 mph), until you find a safe place to stop.
Run-flat tires protect your wheels from damage in case of a loss of tire air pressure. They also protect you from having to stop on the side of the road, which can be quite hazardous. Because it can be difficult to tell if a run-flat tire has lost air pressure, most are installed with a tire-monitoring system .
Run-flat tires are available from several manufacturers, including Goodyear, Dunlop, and Firestone. Michelin also is developing the PAX run-flat tire system, which requires a special wheel, as well.