Getting to Know Your Tires

Posted on May 1, 2008 by Wayne Blanchard

MOST PEOPLE BUY TIRES without ever knowing what the data on the tires mean. When we see a size of 235/70 R 16 101 S, most of us are lost. How do these numbers affect your vehicle? Why are they so important? How do you know that the tire you’re being sold is actually the correct tire for you? Here’s the tire data simplified.

1 • SECTION WIDTH: This (235) indicates the distance (in millimeters) across from the widest point of the sidewall exterior to the widest point of the inner sidewall when mounted on the proper rim. A tire that is too thin may not inflate properly on the rim, and one that is too wide may damage the rim and wheel well.


2 • ASPECT RATIO: This means that the (70) indicates that the height from the sidewall to the tread is 70% of its section width. Basically, the higher the number, the taller the sidewall, and the lower the number, the shorter the sidewall. Too short a sidewall can cause rim damage and create a poor ride. One that is too tall can “rub” and cause damage to surrounding body panels.


3 • TYPE OF CONSTRUCTION: An R indicates that the tire is a radial, which means the tire’s body plies “radiate” out from the center of the tire.


4 • RIM DIAMETER: This (16) is the measurement of the rim from inner bead wall to inner bead wall. Only tires of like size will fit your rims.


5 • LOAD INDEX: This (101) is the maximum weight capacity of your tire. When multiplied times four, you get the total weight of your vehicle and passengers the set of tires can carry. Example: load index of 99 = 1,709 lbs. per tire x 4 = 6,836 lbs. total. For load ratings, see chart below.  


    LOAD INDEX        LOAD (LBS)         LOAD INDEX          LOAD (LBS)    
65 639 85 1135
66 661 86 1168
67 677 87 1201
68 694 88 1235
69 716 89 1279
70 739 90 1323
71 761 91 1356
72 783 92 1389
73 805 93 1433
74 827 94 1477
75 852 95 1521
76 882 96 1565
77 908 97 1609
78 937 98 1653
79 963 99 1709
80 992 100 1764
81 1019 101 1819
82 1047 102 1874
83 1074 103 1929
84 1102 104 1984


6 • SPEED INDEX: This (S) indicates the top speed your tires are rated for. An S rating, for example, indicates a maximum of 112 mph.


Tires that have a higher rating are generally made with a stronger compound and contain a reinforced sidewall. Any tire that is high load rated will most likely have the word “REINFORCED” imprinted into the tire sidewall.


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