Mr. J.R. Singal, M.D. - Eastman Industries
Limited receiving National Export Award from the then Honourable
Prime Minister of India -- Mr.Atal Bihari Vajpayee
Being able to read and comprehend the
information printed on a tire's sidewall will make it easier for you to
understand your tires and assist you in choosing a replacement set.
This first system developed for tire sizing was used until the late 1960s,
but provided only the cross section width of the tire and the rim diameter
in inches. If the section width ended in zero (e.g., 7.00-14 or 7.50-14),
the tire had a common aspect ratio of about 92. For section widths not
ending in zero (e.g., 8.25-15), the tire was considered "low profile"
with an aspect ratio of about 82.
In 1968, a new concept was introduced worldwide. The Alpha-Numeric sizing
system is a load-based system where tires are designated by their
load-carrying capacity and aspect ratio. The first letter is the load and
size relationship, with letters ranging from A to N. The lower the letter,
the smaller the size and, of course, the lower the load-carrying capacity of
To accommodate the smaller tires used on compact cars, the P-Metric
(Passenger Metric) system was created in 1976. The maximum inflation
pressures of P-Metric tires were raised for lower rolling resistance. The
P-Metric system is widely used by domestic tire manufacturers.
Metric Sizing System
The International Standards Organization (ISO) Metric system combines the
Metric system with a service description. The service description provides
the load index along with the speed rating symbol.
The Millimetric sizing system is similar to the Metric system except that
the rim diameter is also represented in millimeters.
Truck Numeric System
Similar to the Numeric system for cars, it lists the section width in
inches, construction type, rim diameter in inches, plus the light truck
Truck Metric Sizing System
Similar to the P-Metric system, except the P is replaced with the LT light
truck designation. Also, LT-Metric and P-Metric tires differ in
Truck High Flotation System
The same as the Light Truck Numeric system with tire diameter added to the
|High Flotation vs. LT-Metric vs. LT-Numeric
referred to as the profile or series, the aspect ratio of a tire is
determined by dividing a tire's section height by its section width when the
tire is: Inflated to maximum air pressure, Mounted on the approved measuring
rim, and Under no load.
lower aspect ratio responds to lateral force more effectively than a tire
with a higher aspect ratio.
The aspect ratio affects steering stability. Generally, the shorter the
sidewall, or the lower the aspect ratio, the less time it takes to transmit
the steering input from the wheel to the tread. The result is quicker
Aspect ratio also affects the tread contact patch. As a rule, a low profile
tire produces a wider tread contact patch. This wider tread contact patch
creates a stiffer footprint that reduces distortion and provides improved
cornering traction. Aspect ratio also impacts ride. A low profile tire
usually has a stiffer ride than the standard aspect ratio of 75 or more.
Many tires come with a service description added on to the end of the
tire's size. These service descriptions contain a two-digit number (load
index) and a letter (speed rating). The load index is a representation of
the maximum load each tire is designed to support. Because the maximum tire
load capacity is branded on the tire's sidewall, the load index is used as a
Use the following chart to determine the maximum load-carrying capacity
based on a tire's load index:
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. Yokohama goes one step further and defines high performance
tires with a speed symbol of "H, V, W, Y," or "Z" and an
aspect ratio of 70 or less (typically, 60 or less). 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. The International
Standard Organization system (ISO) currently serves as a worldwide standard
for tire markings. At the end of a transition period, any speed symbol
denoting a fixed maximum speed capability will be at the end of the service
description following the tire marking (illustrated in the second and third
|*z rating refers to open ended speed
Speed ratings apply only to the tire, not to the vehicle. Putting a speed
rated tire on any car does not mean the car can be operated at the tire's
Ply Rating vs. Load Range
Ply ratings and load ranges identify load and inflation limits of a given
tire size when used in a specific type of service.
Uniform Tire Quality Grade (UTQG) Labeling
- Ply ratings: An older method of rating load capacity, these are
listed as 4-ply, 6-ply, 8-ply, etc.
- Load ratings: The current method of rating a tire's load-carrying
capacity is denoted by letters (B, C, D, E, etc.).
Required by the government, the UTQG provides comparative manufacturer
information. Tires are subjected to a series of government-mandated tests
that measure performance in treadwear, traction and temperature resistance.
All testing is done by the tire manufacturer.
is a measurement of tread durability. Tested against an industry standard,
the assigned numerical grade indicates how well the tread lasts compared
with a reference standard of 100. A treadwear rating 200 means the tread
wears twice as well as the standard. Actual wear depends on the conditions
under which the tire is used. Driving habits, service practices, differences
in road surface and varying climates all affect treadwear.
Traction is a measurement of a tire's ability to stop on wet test surfaces
of asphalt and concrete under controlled conditions. Traction grades are
assigned by the UTQG system and branded on the sidewall. Traction grade is
determined only for straight-ahead, wet braking on concrete and asphalt. It
doesn't include cornering, which may also be an important customer
- Traction Grade A: The tire performed well on both surfaces.
- Traction Grade B: The tire performed well on at least one of the
- Traction Grade C: The tire performed poorly on one or both of the
The UTQG also provides a measure of resistance to heat generation under
normal operating conditions. The test is conducted under predetermined
standards for inflation and loading. Excessive speed, underinflation and
overloading can all cause adverse heat build-up. Sustained high temperatures
can reduce tire durability. Resistance grades are branded on the sidewall.
Department of Transportation (DOT) Certification
- Resistance Grade A: The maximum performance level indicating the tire
withstood a half-hour run at 115 mph without failing.
- Resistance Grade B: The tire passed 100 mph but not 115 mph.
- Resistance Grade C: The minimum performance level indicating that the
tire failed to complete a half-hour at 100 mph.
"DOT" is branded on the tire's sidewall indicating the tire is
certified by the Department of Transportation. Following the DOT branding is
a serial number designating the tire manufacturer, manufacturing plant, tire
size and date of manufacture. Federal law requires that tire dealers record
the DOT identification numbers along with the tire buyer's name and address.
Additional Tire Labeling Conventions Mud and Snow Labeling
If a tire is rated for safe performance in mud and snow, it will be noted
on the sidewall of the tire with either M/S, M+S or M&S. A tire is
certified under the definitions set forth by the Rubber Manufacturers
Tire Construction Labeling
Tread ply and sidewall ply information, including tire ply composition and
materials used, must also be identified. An example would be: Tread: 2 Plies
Rayon + 4 Plies Fiberglass Sidewall: 2 Plies Rayon