Most tire sizes begin with one or more letters :

P = P-Metric (Example: P215/65R17 98T)

• used on passenger vehicles.

Metric / Euro-Metric (Example: 185/65R15 88T)

• usually found on European cars.

LT - Light Truck (prefix) (Example: LT235/75R15 104/101S/C)

• used on SUVs, full-size vans, and medium-duty and heavy-duty pickup trucks with ¾ ton to 1-ton capacity.

LT - Light Truck (suffix) (Example: 9.5-16.5 LT/117R)

• used on light-duty, medium-duty, and heavy-duty pickup trucks (typically ½ ton, ¾ ton or 1-ton load capacity), sport utility vehicles, and vans.

T = Temporary Spare (Example: T/70R17 106M)

• as mini spares or space savers, temporary spares are designed for short-term use until the regular tire is repaired or replaced.

ST - Special Trailer (Example: ST175/80R13)

• special trailer tires and should only be used on car, boat, or utility trailers.

C = Commercial (Example: 31x10.50R15/ C109R)

• used on delivery trucks and vans capable of carrying heavy loads.

Section width of tire (Example: P 225/45R17 91V)

• The three digits following the service type prefix (if present) tell us the cross-sectional width of the tire in millimeters.
• Calculation : dividing the width in millimeters by 25.4

Aspect ratio of sidewall (Example: P225/ 45 91V)

• The two-digit number that usually follows the tire's section width tells us the aspect ratio, or tire profile measurement.
• Calculation : multiplying the sidewall distance in inches by 0.45

Tire and wheel diameter (Example: P225/45R17)

• Tires usually come in the following widths (in inches): 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28.

Unique wheel diameters

• These sizes are usually 14.5, 15.5, 16.5, 17.5, and 19.5 inches, and an example would be 33x12.5R16.5 118R.

Internal construction (example: P225/45R17 91V)

• R means the tire has radial construction
• D means the tire has a bias ply construction
• B means the tire is in belted tires

Speed rating (Example: P225/45R17 91 V)

• The only speed rating still included in the tire size is the Z rating (sports cars)

Service description rating (Example: P225/45R17 91V)

• The service description rating is mandatory for all speed ratings (except Z-rated tires) and appears at the end of the tire's size brand. The service description is used to identify the tire's load index and speed rating

Speed Rating K

• 68 mph
• 110 km/h  

Speed Rating L

• 75 mph
• 120 km/h  

Speed Rating M

• 81 mph
• 130 km/h  

Speed Rating N

• 87 mph
• 140 km/h
• Used in temporary spare tires

Speed Rating P

• 93 mph
• 150 km/h  

Speed Rating Q

• 99 mph
• 160 km/h
• Used in studless and studded winter tires

Speed Rating R

• 106 mph
• 170 km/h
• Used in heavy duty light truck tires

Speed Rating S

• 112 mph
• 180 km/h
• Used in family sedans and vans

Speed Rating T

• 118 mph
• 190 km/h
• Used in family sedans and vans

Speed Rating U

• 124 mph
• 200 km/h
• Used in sport sedans and coupes

Speed Rating H

• 130 mph
• 210 km/h  

Speed Rating V

• 149 mph
• 240 km/h
• Used in sport sedans, coupes and sports cars

Speed Rating W

• 168 mph
• 270 km/h
• Used in exotic sports cars

Speed Rating Y

• 186 mph
• 300 km/h
• Used in exotic sports cars

Speed Rating Z

• 149+ mph
• 240+ km/h
• Used in sports cars