Passenger Tire sizing is typically displayed as:

P215/65R15 89H   

• The "P" stands for "P-Metric" or "Passenger". This means that it is a North American tire sizing designation. European tires typically don't have the "P" attached to the size. Tires with higher ply ratings will generally start with "LT" which stands for "Light  Truck". This indicates the tire is an LT metric and will always have a Load Range indicated. It is important to note this for vehicles that call for LT metric tires. Never substitute a P metric tire for an LT metric tire, even if all the other dimensions are the same.
• The "215" is the width of a tire, also known as the "section width". This is the width of the tire in millimeters at its widest point from sidewall to sidewall when mounted on the recommended rim width. The actual tire width can vary depending on the rim width it is mounted on.
• The "65"is known as the Aspect Ratio. It is calculated by dividing the section height by the section width and multiplying by 100. (In this example, the sidewall will be 65% of 215)
• The "R" stands for Radial, meaning it has a radial construction. Radial tires have ply cords that extend to the beads and are laid at 90 degrees to the centerline of the tread, the carcass being stabilized by a circumferential belt. Other possibilities include "B" for belted construction and "D" for diagonal construction. This means the ply cords extend to the beads and are laid at alternate angles less than 90 degrees to the centerline of the tread.
• The "15" stands for the diameter of the wheel in inches. This is the exact size that this tire will fit. There are some older rims called "TRX" which are metric measurements like 390. You CAN NOT mix TRX rims with regular tires or vise-versa.
• The "89" is the load index
• The "H" is the speed symbol.

The speed rating of any tire is a measurement of the top safe speed the tire can carry a load under specified conditions. It is also an indication of how the tire will handle at lower speeds. A higher rated tire will give you better traction and improved steering response even at 50mph.

Below is a listing of common speed ratings:

Q = 99 MPH, 160km/h
S = 112 MPH, 180km/h
T = 118 MPH, 190km/h
U = 124 MPH, 200km/h
H = 130 MPH, 210km/h
V = 149 MPH, 240km/h
Z = 149 MPH, 240km/h and over
W = 168 MPH, 270km/h
Y = 186 MPH, 300km/h

It is not recommended to downgrade your tires to a lower speed rating than of the OEM tires that came with your vehicle at the time of purchase. Doing so can effect the handling of the car, and in an emergency situation that can be dangerous. Also, never mix tires with different speed ratings on your vehicle.

Below is a quick legend to notations describing the appearance of the tire's sidewall.

Marking               Meaning

BCS                       BLACK CIRCUMFERENCIAL SERRATION
BL                         BLACK LETTERS
BSL                       BLACK SERRATED LETTERS
BSB                       BROKEN SERRATED BAND
ENWL                    EXTRA NARROW WHITE LETTERS
ROBL                     RAISED OUTLINED BLACK LETTERS
OWL                      OUTLINED WHITE LETTERS
OBL                       OUTLINED BLACK LETTERS
OGL                       OUTLINED GOLD LETTERS
ORBL                     OUTLINED RAISED BLACK LETTERS
ORWL                    OUTLINED RAISED WHITE LETTERS
RBL                        RAISED BLACK LETTERS
RWL                       RAISED WHITE LETTERS
RRBL                      RECESSED RAISED BLACK LETTERS
SBL                        SERRATED BLACK LETTERS
SRBL                      SERRATED RAISED BLACK LETTERS
SOWL                     SLANTED OUTLINED WHITE LETTERS
SVSB                      SLANTED VERTICAL SERRATED BAND
VSB                        VERTICAL SERRATED BAND
WL                         WHITE LETTERS
WS                         WHITE STRIPE
WW                        WHITE WALL

The Uniform Tire Quality Grading rating is a quality rating system developed by the American Department of Transportation. It is designed to tell consumers the relative performance of passenger tires (but does not apply to winter  tires).

Below is an example of a UTQG Rating:

150A B

The "150" indicates the treadwear rating
the "A" indicates traction
the "B" indicates temperature

The Treadwear rating is based on a wear test performed on a 400 mile government test course covering specified sections of public roads in Texas. A group of not more than 4 test vehicles travels the course in a convoy so that all tires experience the same conditions. Tread groove depths of the tires being tested are measured after each 800 miles. The same procedure is followed for a set of "control" or "course monitoring tires" Upon the completion of the 7200 mile test, the rating results of both tires are compared, and the tires being tested are assigned a treadwear rating according to government standards. This number can be used to compare between tires. In the above example, this tire rated 150 should last 1/2 as long as a tire rated 300. The relative performance of tires depends upon the actual conditions of their use and may be significantly different from the norm due to differences in road characteristics and climate.

The Traction ratings are AA, A, B, and C, from highest to lowest. This measurement indicates a tires ability to stop in a straight line on wet pavement. The rating is based on a 40mph test where the brakes are "locked up" on wet asphalt and wet concrete surfaces. It's important to remember that this rating does not indicate the tires ability to resists hydroplaning, and do not apply to cornering traction.

Temperature ratings are determined by running tires on an indoor roadwheel test under specified conditions. Successive 30 minute runs are made in 5mph increments starting at 75mph and continuing until the tires fails. Tires with an A rating must withstand at least 114mph, B at least 99mph, and C at least 85mph. These measurements indicate the tire's ability to sustain high temperatures which often cause tires to wear away quickly or in extreme conditions lead to sudden tire failure.

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