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 tirePlus zeroPlus onePlus two
225-55-16245-50-16245-45-17245-40-18

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 IndexLoad (lbs.)Load IndexLoad (lbs.)Load IndexLoad (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

Speed-rated tires are identified by letters such as P, Q, R, S, T, H, V, W, Y, (Y), or Z as either part of the size designation (e.g. ZR) or part of the service description adjacent to the size designation (e.g. 94H). The letter indicates the maximum speed capability of the tire when properly loaded and inflated. However, even when tires are properly loaded and inflated, driving for prolonged period at speeds greater than posted limits can cause tire damage and possibly tire failure, which could lead to an accident. Original Equipment speed-rated tires must be replaced with tires of the same or higher speed rating if the speed capability of the vehicle is to be maintained.

Speed symbols may currently be marked on a tire in any of three ways: 205/60ZR15; 205/60ZR15 89W. The International Standard Organization (ISO) system currently serves as a worldwide standard for tire markings. At the end of a transition period, any speed symbol denoting a fixed maximum speed capability will be at the end of the service description following the tire marking.

Speed rating codes

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.

Additional marks

There are numerous other markings on a typical tire, these may include:

  • M+S - Mud and Snow; winter/all-weather tires with above-average traction in muddy or snowy conditions. Spike tires have an additional letter, "E" (M+SE).
  • BSW - Black SideWall
  • WSW - White SideWall
  • E4 - Tire approved according ECE-regulations, the number indicating the country of approval.
  • 030908 - Approval number of the tire
  • DOT code - All tires imported into the USA have the DOT code, as required by the Department of Transportation (DOT). It specifies the company, factory, mold, batch, and date of production (2 digits for week of the year plus 2 digits for year; or 2 digits for week of the year plus 1 digit for year for tires made prior to 2000)
  • TL - Tubeless
  • TT - Tube-type, tire with an inner-tube
  • Made in - Country of production
  • C - Commercial; tires for light trucks (Example: 185 R14 C)
  • B - Bias belted; tires for motorcycles (Example: 150/70 B 17 69 H)=diagonal construction with belt added under the tread
  • SFI - Side Facing Inwards; inside of asymmetric tires
  • SFO - Side Facing Outwards; outside of asymmetric tires
  • TWI - Tread Wear Indicator; a device in the main tire profile which shows when the tire is worn down and needs to be replaced
  • SL - Standard Load; tire for normal usage and loads
  • XL - eXtra Load; tire for heavy loads
  • RF - Reinforced tires
  • Arrows - Some tread designs are designed to perform better when driven in a specific direction (clockwise or counter-clockwise). Such tires will have an arrow showing which way the tire should rotate when the vehicle is moving forwards. It is important not to put a 'clockwise' tire on the left hand side of the car or a 'counter-clockwise' tire on the right side.

To facilitate proper balancing, most high performance tyre manufacturers in addition place red (uniformity) and yellow (weight) marks on the sidewalls of its tyres to enable the best possible match-mounting of the tyre/wheel assembly.

Example:

The tires on a Mini Cooper might be labeled:

P195/55R16 85H

  • 'P' these tires are for a passenger vehicle.
  • 195 – the width of the tire is 195 mm at the widest point.
  • 55 – indicates that the height of the side-wall of the tire is 55% of the width - 107 mm.
  • R – this is a radial tire.
  • 16 – this tire fits 16 inch diameter wheels.
  • 85 – the load index, a maximum of 515 kg per wheel in this case.
  • H – the speed index, this means the maximum permitted speed, here 210 km/h (130 mph).

The tires on a Chevy Suburban might be labeled:

LT245/75R16

Uniform Tire Quality Grading (UTQG)

The National Highway Traffic Safety Administration (NHTSA) established the Uniform Tire Quality Grading Standards (UTQGS) in 49 CFR 575.104.
Required by the U.S. Government, the UTQG provides comparative manufacturer information. Tires are subjected to a series of government-mandated test that measure performance in treadwear, traction and temperature resistance.

Components

The UTQG rating is made up of 3 components:

Treadwear

The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test track. A tire graded 200 would wear twice as long on the government test course under specified test conditions as one graded 100. It is wrong to link treadwear grades with your projected tire mileage. The relative performance of tires depends upon the actual conditions of their use and may vary due to driving habits, service practices, differences in road characteristics and climate.

Traction

Traction grades, from highest to lowest, are AA, A, B and C. They represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete.

  • Traction Grade AA: Highest rating in the UTQG system.
  • Traction Grade A : The tire performed well on both surfaces.
  • Traction Grade B : The tire performed well on at least one of the surfaces.
  • Traction Grade C : The tire performed poorly on one or both of the surfaces.

Temperature

The temperature grades, from highest to lowest, are A, B and C. These represent the tire's resistance to the generation of heat. The test is conducted under predetermined standards for inflation and loading. Excessive speed, under inflation and overloading can all cause adverse heat build-up. Sustained high temperatures can reduce tire durability. Resistance grades are branded on the sidewall.

  • Resistance Grade A: The maximum performance level indicating the tire withstood a half hour run at 115 mph without failing.
  • Resistance Grade B: The tire passed 100mph but not 115 mph.
  • Resistance Grade C: The minimum performance level indicating that the tire failed to complete a half-hour at 100 mph.

Department of Transportation (DOT) Certification

“DOT” is branded on the tire’s sidewall indicating the tire meeting all standards of the U.S. Department of Transportation. Following the DOT branding is a serial number designating the tire manufacturer, manufacturing plant, tire size and date of manufacture. Federal law requires that tire dealers provide customers tire registration forms and instructions on how to register tire information with the tire manufacturer. This is done to facilitate customer notifications in case of a product safety campaign or tire recall.

PLY RATING vs LOAD RANGE

Ply ratings and load ranges identify load and inflation limits of a given tire size when used in a specific type of service.
  • Ply Ratings: An older method or rating load-carrying capacity, these are listed as 4-ply, 6-ply, 8-ply, etc.
  • Load Ratings: The current method or rating a tire’s load-carrying capacity is denoted by letters (B, C, D, E, etc.)

The following chart describes the relationship between a tire’s ply rating and the corresponding load range:

Ply RatingLoad Range
4
6
8
10
12
14
B
C
D
E
F
G