The amount of air inside the tire pressing outward on each
square inch of its surface; expressed in pounds per square
inch. For cars and light trucks, the air pressure measured
when the tire is cold (before the vehicle is driven) establishes
its recommended air pressure setting.
The mechanical condition of adjustable components within
the vehicle's suspension. When a vehicle is in alignment,
the caster, camber, toe-in and thrust settings are
set to specification. Severe impacts (hitting potholes
or curbs) and worn suspension parts are the leading
causes of misalignment.
All Season Tires
Tires designed to provide good traction in a wide variety
of road conditions, including wet, dry and mud and snow.
This design also limits the tire's performance in extreme
conditions, or when compared to tires built for a particular
A term for describing the size of a tire (H78-15, for example)
where both letters and numbers are used.
A term that describes a tire's height-to-width proportion.
If a tire's sidewall height were 65% of its section width,
its aspect ratio would be 65. In the tire size expressed
as 205/65-15, the number 65 is the aspect ratio.
The state in which a tire and wheel assembly spins with
all its weight distributed equally. A wheel balancer is
used to place weights compensating for static and dynamic
imbalances that exist in all assemblies. Not balancing an
assembly can result in vibration.
(Tire Balancing article
A round hoop of steel wires, wrapped or reinforced
by steel cords, placed at the very inside of
the tire's diameter.
(Tire Construction article
Bias Ply Tire
A pneumatic tire manufactured such that the plies
are laid at alternate angles less than 90 degrees
to the centerline of the tread. These criss-cross
plies give the tire its strength, but generate heat
during operation and limit the tire's wear and
The tire body beneath the tread and sidewalls; also called the casing.
The portion of the tread that contacts the road during operation.
The strands of material forming the plies or layers of
tire. Cords may be made from fiberglass, rayon, nylon,
polyester or steel.
Each tire has a required Department of Transportation
number imprinted on at least one of its sidewalls.
That number begins with the letters "DOT" and may
contain up to 12 additional numbers and letters.
The first and last digits are the most important
for the tire owner. The first two letters/numbers identify
the manufacturer of the tires. Prior to the year
2000, the last three digits of a DOT number represented the
week (two digits) and the year (one digit) of production.
For example, if the last three digits are 439, the
tire was produced in the 43rd week of 1999. Tires
produced after January 1, 2000 have a four digit date
code at the end of the DOT number. The first two
digits represent the week of production and the last
two digits represent the last two digits of the year
of production. So, 3500 as the last four numbers
indicates that the tire was produced in the 35th week
of the year 2000.
The portion of the tire that makes contact with the surface of the road.
The resistance of one material (the tire tread) as it
moves against another (the road); this is the force
that causes the tire to grip to the road.
Gross Vehicle Weight
The actual weight of a vehicle when fully loaded with passengers and cargo.
The space between two adjacent tread ribs; also called tread grooves.
A method of "breaking in" competition tires prior to
initial use. Heat cycling gradually heats the tire in a
controlled environment to gently stretch the tread compound,
resulting in better traction and longer tread life.
Also called summer tires; designed for dry and occasional
wet weather driving, but not for use on snow and ice.
A skimming effect caused by tires losing contact with
a surface covered by water.
The innermost layer of a tubeless tire which prevents air
from permeating through the tire. This thin layer of
material replaces the innertube.
An assigned number ranging from 0 to 279 that corresponds
to the load carrying capacity of the tire.
(Load Index chart
M+S, M/S or M & S (Mud and Snow)
Indicates that a tire can reach particular standards
for performance in mud and snow conditions. The tire
must meet the Rubber Manufacturers Association (RMA)
definition of a mud and snow tire.
Maximum Inflation Pressure
The maximum air pressure to which a cold tire may
be inflated; found molded onto the tire's sidewall.
OE and OEM
OE means "Original Equipment" and refers to the tires
included with a new vehicle at the time of purchase.
The vehicle's manufacturer selects these tires to
provide the optimal performance based on the performance
characteristics of the vehicle. "OEM" stands for
"Original Equipment Manufacturer."
The diameter of an inflated tire without any load.
The distance between a tire's outside sidewalls, including
lettering and designs.
Uniform designation of tire sizes in metric measurements
originally introduced by American tire manufacturers
in 1977. Commonly called "P-metric series." A typical
P-metric tire size is P215/70R-15.
A small label typically located on the edge of the driver's
door or inside the glove compartment of a vehicle. A
placard contains information on the vehicle such as the
manufacturer's recommended tire inflation pressure, seating
capacity, and Gross Vehicle Weight (GVW).
An option allowing drivers to customize the appearance
and performance of their vehicle by mounting lower profile
tires on larger diameter wheels. One-inch greater wheel
diameter is referred to as plus-one, two inches is
plus-two... and so on. Using a lower profile tire with a
greater diameter rim allows the overall diameter to remain
about the same.
(Plus Sizing article
A rubber-coated layer of fabric containing cords that run
parallel to each other; extends from bead to bead and
goes between the innerliner and belts of tread.
This letter indicates the load carrying capacity of the tire in terms of its
construction. A "C" indicates the tire has a 6-ply load carrying capacity.
The tire is not actually built with 6 plies, but contains one or two plies
of equivalent strength. A "D" is an 8-ply rating, and an "E" is a 10-ply
rating. If there is no letter, the tire has a standard 4-ply rating.
Pounds Per Square Inch. This is the standard unit of
measurement for air pressure within tires.
Tire construction where the cords in the body run at
90 degrees to the centerline of the tread.
A system of balancing a tire and wheel assembly using a
simulated road test. Ride matching provides optimal
weight distribution and eliminates vibrations caused by
the combination of minor errors within tires and wheels.
(Ride Matching article
Distance between the two opposite inside edges of the rim flanges.
The force required to keep a tire moving at a constant
speed. The lower the rolling resistance, the less energy
needed to keep a tire moving.
Moving tires from side to side or front to rear on a vehicle in a
prescribed pattern to achieve uniform wear on all tires. Rotations
should be performed regularly every 6,000 miles.
(Tire Rotation article
A numerical representation of a tire's aspect ratio. For example,
60 Series indicates the tire's section height is 60% of its section
width (See Aspect Ratio).
Wobbling of wheels from side to side on a vehicle. Improperly
balanced tires, misalignment and bent wheels can cause shimmying.
The part of a tire where the sidewall and tread meet.
Certain tire design features shoulder blocks for better
The part of the tire between the tread and the bead.
An expression that defines
a particular tire in terms of its width, height, rim diameter, aspect
ratio and construction type. 205/65R-15 expresses tire size using the
metric system. For more detailed information, visit our page on
reading the tire size.
(Reading the Tire Size article
Also referred to as a
winter tire; a special type of tire with a tread and compound that
gives better traction in snow. Identified by M&S, M+S or M/S on the
sidewalls. All season tires also include these designations on the
sidewall. (Winter Tire FAQ
The speed rating of a
tire is based on U.S. Government standards for reaching and sustaining
a specified speed. Typically, a tire with a higher speed rating results
in better handling. Speed ratings are determined via laboratory tests
that simulate road performance at various speeds. Tires are assigned a
single letter (such as H or V) to designate speed rating.
(Speed Rating article
Tire Pressure Monitoring System (TPMS)
TPMS is an automated system that monitors the air pressure in a vehicle's
tires. When air pressure in one or more tires drops 25 percent or more
below the correct pressure, a warning alerts the driver.
The friction between a
tire and the road surface; the amount of grip provided.
The part of the tire that
comes into contact with the road. The tread type is distinguished
by the design of its ribs and grooves.
(Tire Construction article
The distance measured
in the major tread groove nearest the centerline of the tire from the
base of the groove to the top of the tread. According to law, most
states legally consider a tire to be worn out when it reaches a
tread depth of 2/32".
sometimes called "wear bars", that appear across the tread when
2/32" of tread remains.
The width of a tire's tread.
UTQG (Uniform Tire Quality Grading)
A tire information system that provides consumers with ratings for
a tire's traction (AA to C) and temperature (A to C). Treadwear is a
numeric rating. Ratings are determined by tire manufacturers using
government-prescribed test procedures, and are molded into the tire's
sidewall. These ratings can only be compared within specific
manufacturer's tires and cannot be compared from one manufacturer to
another. Our treadwear, traction and temperature page explains this
rating system in much more detail.
(Treadwear, Traction, and Temperature article
A device mounted in the
wheel that lets air in or out of the tire. Valves include caps to
keep out dirt and moisture and a valve to prevent air from escaping.