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Tire Size

Explanation of tire size codes

Passenger Vehicles

The ISO Metric tire code consists of a string of letters and numbers, as follows:

  • An optional letter (or letters) indicating the intended use or vehicle class for the tire:-
    • P: Passenger Car
    • LT: Light Truck
    • ST: Special Trailer
    • T: Temporary (restricted usage for "Space-Saver" spare wheels)

      Use of the letter P indicates that the tire is engineered to TRA standards and absence of a letter indicates that the tire is engineered to ETRTO standards. In practice, the standards of the two organizations have evolved together and are virtually interchangable.[5]
    • 3 digit number: The "nominal section width" of the tire in millimeters; the widest point from both outer edges.
    • /: Slash character for character separation.
    • 2 or 3 digit number: The "aspect ratio" of the sidewall height to the total width of the tire, as a percentage. If the information is omitted, it is assumed to be 82%. If the number is larger than 200, then this is the diameter of the entire tire in millimeters.
    • An optional letter indicating construction of the fabric carcass of the tire:
      • B: bias belt (where the sidewalls are the same material as the tread, leading to a rigid ride)
      • D: diagonal
      • R: radial
        • if omitted, then it is a cross ply tire
    • 2 digit number: Diameter in inches of the wheel that the tire is designed to fit. There is the rare exception metric diameter tires, such as the use of the 390 size, which in this case would indicate a wheel of 390 mm in diameter. Few tires are made to this size presently.[6]
    • 2 or 3 digit number: Load index; see table below
    • 1 or 2 digit/letter combo: Speed rating; see table below
    • Additional marks: See subheading below.

Light trucks

Some light truck tires follow the Light Truck Numeric or Light Truck High Flotation systems, indicated by the letters LT at the end instead of the beginning of the sequence, as follows:

  • The tire diameter is given for High Flotation tires and omitted from Numeric tires.
    • 2 digit number: The diameter of the tire in inches.
    • x: Separator character.
  • 3 or 4 digit number: The section width (cross section) of the tire in inches. If the tire diameter is not given, section widths ending in zero (e.g., 7.00 or 10.50) indicate the aspect ratio is 92%, while section widths not ending in zero (e.g., 7.05 or 10.55) indicate the aspect ratio is 82%.
  • Construction of the fabric of the tire:
    • B: bias belt
    • D: diagonal
    • R: radial
  • 2 digit number: Diameter in inches of the wheel rim that this tire is designed to fit.
  • LT: Designates that this is a Light Truck tire.
  • Load index and speed rating are not required for this type of tire but may be provided by the manufacturer.

Load index

The load index on a passenger car tire is a numerical code stipulating the maximum load (mass, or weight) each tire can carry. For Load Range "B" tires, ETRTO (ISO-Metric) standards specify the load index rating at an inflation pressure of 36 psi while P-Metric standards measure the load capacity at an inflation pressure of 35 psi. The two standards vary slightly with the capacity required for different inflation pressures.

While all ETRTO tires of the same load index will have the same maximum load (in pounds), P-Metric tires with the same load index may have different load capacities depending on the tire size. The TRA Inflation Tables must always be consulted when comparing the load capacity of P-Metric tires -- the Load Index alone is not sufficent. An example: a P205/50R15 Standard Load tire has a load index of 84 and a load rating of 1113 pounds at 35 PSI. A P215/50R13 with the same load index of 84 only has a load rating of 1091 pounds, also at 35 PSI.

Standard Load Inflation Table for ETRTO Tires
Code Pounds Kilograms Code Pounds Kilograms Code Pounds Kilograms Code Pounds Kilograms
60 551 250 80 992 450 100 1,764 800 120 3,087 1,400
61 567 257 81 1,019 462 101 1,819 825 121 3,197 1,450
62 584 265 82 1,047 475 102 1,874 850 122 3,306 1,500
63 600 272 83 1,074 487 103 1,929 875 123 3,418 1,550
64 617 280 84 1,102 500 104 1,984 900 124 3,528 1,600
65 640 290 85 1,135 515 105 2,039 925 125 3,638 1,650
66 661 300 86 1,168 530 106 2,094 950
67 677 307 87 1,201 545 107 2,149 975
68 695 315 88 1,235 560 108 2,205 1,000
69 717 325 89 1,279 580 109 2,271 1,030
70 738 335 90 1,323 600 110 2,337 1,060
71 761 345 91 1,356 615 111 2,403 1,090
72 783 355 92 1,389 630 112 2,470 1,120
73 805 365 93 1,433 650 113 2,536 1,150
74 827 375 94 1,477 670 114 2,601 1,180
75 853 387 95 1,521 690 115 2,679 1,215
76 882 400 96 1,565 710 116 2,756 1,250
77 908 412 97 1,609 730 117 2,833 1,285
78 937 425 98 1,653 750 118 2,910 1,320
79 963 437 99 1,709 775 119 2,999 1,360

 

Speed rating

The code is made up of one or two letters, or one letter and one number. It indicates the maximum permitted speed that the tire can sustain for a ten minute endurance without being in danger.

Speed rating
Code mph km/h Code mph km/h
A1 3 5 L 75 120
A2 6 10 M 81 130
A3 9 15 N 87 140
A4 12 20 P 94 150
A5 16 25 Q 100 160
A6 19 30 R 106 170
A7 22 35 S 112 180
A8 25 40 T 118 190
B 31 50 U 124 200
C 37 60 H 130 210
D 40 65 V 149 240
E 43 70 Z over 149 over 240
F 50 80 W 168 270
G 56 90 (W) over 168 over 270
J 62 100 Y 186 300
K 68 110 (Y) over 186 over 300

Prior to 1991, tire speed ratings were shown inside the tire size, before the "R" character. The available codes were SR (112 mph, 180 km/h), HR (130 mph, 210 km/h), VR (in excess of 130 mph, 210 km/h) and ZR (in excess of 149 mph, 240 km/h). The ZR code is still in common use, often combined with a W or Y speed code.

Tires with a speed rating higher than 186 mph (300 km/h) are indicated by a Y in parenthesis. The load rating is often included within the parenthesis, e.g. (86Y).