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Wheel & Tire Conversion Specialist

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Tire Tech

HOW TO READ A TIRE’S SIDE WALL

Being able to read and understand the information molded on a tire?s sidewall will make it easier for you to answer questions and make better assessment when buying tires for you car.

TIRE SIZING

Tire Sizing is also known as “Plus Sizing” on a tire. “Plus sizing” refers to changing to a shorter tire sidewall without changing the overall diameter of the tire, in order to maintain speedometer accuracy and not upset other systems such as traction control. The number after the plus indicates how many inches larger the wheel diameter is. For example, “Plus one” means that the new wheel is one inch larger than the original wheel.
PLUS SIZING EXAMPLE
Stock tire Plus zero Plus one Plus two
225-55-16 245-50-16 245-45-17 245-40-18

Advantages to Plus Sizing

Advantages may include greater handling and cornering abilities of the car, but this is more a result of the wider tread face and stiffer sidewall of the plus size tires, and can be achieved with tires that are simply stiffer and wider instead of plus sized. Wider tires may decrease braking distances on dry pavement, but see the section on the disadvantages of plus sizing to learn about the physics of plus sizing and the factors that may realistically cancel the benefits of wider tires out.

There is a subjective “appeal” of larger wheels, as in the eyes of some individuals larger wheels are more appealing.

Disadvantages to Plus Sizing

Besides from the obvious cost difference, plus sizing past Plus 1 will seldom give you much added performance. Additionally, the tread life of the tire is much more likely to wear out quicker and many vehicles will need a new set of wheels to match the new set of tires. Larger tires are also much easier to damage, more susceptible to hydroplaning (riding the surface of the water), and less effective in wet weather.

Plus sizing also does not enhance the value of the vehicle (and in some cases diminishes it, since a new set of wheels means the vehicle is not on par with original factory recommended specifications along with not having original factory parts) and SUV/Truck owners will be more vulnerable to rollovers. Ride comfort will also have to be sacrificed in order to deal with performance enhancement.

Furthermore, despite what has been said in other portions of this entry, plus sizing may significantly decrease the acceleration, braking, and fuel economy of a vehicle. Even though the overall wheel diameter measured at the tread may stay the same, the combination of a moving the heavy rim component of a wheel outwards from the center of rotation of the wheel results in a significant increase in rotational mass for each wheel, assuming the same materials are used for the construction of the two wheels. This increase can result in a measurable increase in the amount of energy needed to accelerate or decelerate each wheel. The corresponding result is an increase in acceleration times and stopping distances. In other words the car gains and loses speed slower. Like all components in any machine, moving weight from one location of a part to another location of a part where that weight has to be moved through a larger distance will result in reduced response times in regards to altering its rate which is otherwise knows as it?s speed. Any change of this manner will have a detrimental overall effect on a vehicles performance.

By replacing a heavier, steel wheel with a lighter aluminum allow wheel, the aforementioned performance degradation due to increased rotational inertia can be offset. In fact, it is often the case that an overall decrease in tire-wheel mass can be achieved by plus-sizing, particularly on older, or economy cars where steel wheels are more prevalent.

LOAD INDEX

The Load Index is an assigned number that corresponds to the load-carrying capacity of a tire. Using the following example, tire size 205/60R15 91V, the load index 91 corresponds to a load-carrying capacity of 615 kg (1356 lbs) at maximum inflation pressure. The speed symbol of the tire is the second part of the service description.

205/60R15 91V
205 = Section Width in Milimeter
60 = Aspect Ratio
R = Radial Construction
15 = Rim Diameter in Inches
Service Description [91V]
91 = Load Index
V = Speed Symbol
206

The Service Description system is used with most tires – except “Z” when no service description is given. For “Z” rated tires without a service description is given. For “Z” rated tires without a service description, consult the vehicle manufacturer for maximum speed. For proper load-carrying capacity, consult the “High-Speed Driving for Passenger Tires” section of this Guide. For European Metric tires (non P-Metric), the European Tyre and Rim Technical Organization (ETRTO) load values are defined in a load index table (see below) and valid for European Metric tire sizes. For P-Metric tires, the tire load capability is based upon a dimension specific formula that can result in different load carrying capability within the same load index. Please note that European and P-Metric sizing systems within the same size can have different load indices. Therefore, for P-Metric tires the maximum load can be found on the tire sidewall.

ETRTO Load Index and Equivalent Loads
Load Index Load (lbs.) Load Index Load (lbs.) Load Index Load (lbs.)
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
827
853
882
908
937
963
992
1019
1047
1074
1102
1135
1168
1201
1235
1279
1323
1356
1389
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
1433
1477
1521
1565
1609
1653
1709
1764
1819
1874
1929
1984
2039
2094
2149
2205
2271
2337
2403
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
2469
2535
2601
2679
2756
2833
2910
2998
3086
3197
3307
3417
3527
3638
3748
3858
3968
4079
4189

SPEED RATING

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

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

VR

over 130 over 210

E

43 70

V

149 240

F

50 80

Z

over 149 over 240

G

56 90

W

168 270

J

62 100

Y

186 300

K

68 110

ZR

over 186 over 300

Codes VR and ZR appear in place of R in the size designation. Still, a maximum speed letter may appear after the weight code placing a corresponding limit.

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