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Tire Sizing

Passenger and Light Truck Tire Sizing

Passenger & Light Truck Tire Sizing

Passenger Tire Sizing ( 5 Articles )
Four Sizing systems exist for passenger tires today:

P-Metric
European Metric
Millimetric
Alpha-Numeric

Each of these systems evolved from the first tire sizing system - the Numeric Sizing system - which is now obsolete. It was developed when all tires had the same aspect ratio, and it provided only the nominal cross section width of the tire and the rim diameter in inches. The following are examples that identify the three sizing systems that are commonly seen today.
Light Truck Tire Sizing ( 2 Articles )
Sizing for light truck tires takes the performance requirements of the vehicle, and the tires, into account. Light truck tires have evolved along with the expanded applications of trucks and vans which have grown to be multi-purpose vehicles that we use for work, for play or as passenger vehicles. There are three different light truck tire sizing systems: Light Truck Metric, Light Truck High Flotation and Light Truck Numeric.

Light Truck Metric

This sizing system mirrors the P-Metric system for passenger tires.

LT235/75R15/C
LT = Light Truck Designation
235 = Section Width in Millimeters
75 = Aspect Ratio
R = Radial Construction
15 = Rim Diameter in Inches
C = Load Range

Alpha-Numeric

In the late sixties, this load-based system evolved. The first letter of the designation identifies the load/size relationship of the tire. The letter can range from "A" to "N", with "A" being the lowest. The lower the letter, the smaller the size and load-carrying capacity at a given air pressure.

BR60-13
B = Load-Carrying Capacity
R = Radial Construction
60 = Aspect Ratio
13 = Rim Diameter in Inches

Millimetric

240/55R390
240 = Section Width in Millimeters
55 = Aspect Ratio
R = Radial Construction
390 = Rim Diameter in Millimeters

European Metric

Essentially this system was a conversion of the Numeric system from inches to millimeters. Aspect ratio appears in the size designation in most cases where it is other than 82.

155SR13, 155R13, 155R13 78S 155 = Section Width in Millimeters
S = Speed Rating (See pages 7&8)
R = Radial Construction
13 = Rim Diameter in Inches
78S = Service Description (Load Index and Speed Rating)

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