Passenger Truck
PASSENGER AND LIGHT TRUCK TIRE SIZING
Passenger Tire Sizing
Four Sizing systems exist for passenger tires today: P-Metric, European Metric, Millimetric, and 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.
P-Metric
Today, most U.S. tire manufacturers build tires that conform to this system. It evolved in the late seventies in an attempt to standardize tire sizing worldwide and is based on the metric system.
P215/65SR15, P215/65R15 P215/65R15 95S
P = Passenger Car Tire
215 = Section Width in Millimeters
65 = Aspect Ratio
R = Radial Construction
15 = Rim Diameter in Inches
95S = Service Description (Load Index and Speed Rating)
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)
Millimetric
240/55R390
240 = Section Width in Millimeters
55 = Aspect Ratio
R = Radial Construction
390 = Rim Diameter in Millimeters
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
Light Truck Tire Sizing
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
Light Truck High Flotation
Light truck high flotation tires evolved in the mid-seventies as lower aspect ratios became more popular on light trucks and vans. The combination of lower aspect ratios and high flotation yielded better traction on sand and soft soil found in water and off-road situations.
31X10.50R15LT/C
31 = Tire Overall Diameter in Inches
10.50 = Section Width in Inches
R = Radial Construction
15 = Rim Diameter in inches
LT = Light Truck Tire
C = Load Range
Light Truck Numeric
This older system is still widely used, mostly on commercial vehicles.
9.50R16.5LT/D
9.50 = Section Width in inches
R = Radial Construction
15 = Rim Diameter in Inches
LT = Light Truck Designation
D = Load Range
SPEED RATING SYSTEM
The speed rating of a tire indicates the speed category (or range of speeds) at which the tire can carry a load under specified service conditions. The speed rating system used today was developed in Europe in response to the need to control the safe performance of tires at standardized speeds. A letter from A to Z symbolizes a tire's certified speed rating, ranging from 5 km/h (3 mph) to above 300 km/h (186 mph). This rating system describes the top speed for which a tire is certified. It does not indicate the total performance capability of a tire.
When this speed rating system was originally developed, the Unlimited V category of over 210 km/h (130 mph) was the top speed rating a tire could achieve.
As manufacturers made more tires that fit into this category, it was necessary to better regulate performance at standardized speeds to ensure safety. The Limited V category of 240 km/h (149 mph) was then created, and the Z speed rating was added as the top speed rating that a tire could achieve. Recently, W and Y limited speed symbols have been added as higher speed categories.
Always consult the manufacturer for the maximum speed of Unlimited Z tires. Speed rating is identified as a part of the tire's sizing or service description.
In the latest attempt to standardize tire designations, all ratings except unlimited ZR incorporate the speed symbol and load index as the tire's service description. For example:
205/60R15 91V
205 = Section Width in Millimeters
60 = Aspect Ratio
R = Radial Construction
15 = Diameter in Inches
91 = Load Index
V = Speed Symbol Service Description
"ZR" Rated Tires
When "ZR" appears in the size designation with the service description, the maximum speed is as indicated by the service description.
EXAMPLES:
|
Tire Designation P275/40ZR17 |
Maximum Speed 93W 270 km/h (168 mph) |
For tires having a maximum speed capability above 240 km/h (149 mph), a "ZR" may appear in the size designation. For tires having a maximum speed capability about 300 km/h (186 mph), a "ZR" must appear in the size designation. Consult the tire manufacturer for maximum speed when there is no service description.
Speed Symbols
|
Speed Symbol A1 |
Speed (km/h) 5 |
Speed (mph) 3 |
