Identifying the size of tires on your car

Sidewall Markings

On the sidewall of a tire you will find various codes and markings. Amongst these you will find the size.

Tire Sizes

Tire sizes are made up of a number of different numbers and letters. For example tire size 185/60 R 14 82 H is made up of the following information:

185 The tire section width in millimetres
60 The aspect ratio in % (the height of the sidewall divided by the tire's width)
R Denotes the tire's construction type - in this case it's a radial
14 Rim diameter in inches
82 Load Index
H Speed Rating

This is all the information you need to order your tires if you would like to know more read on

Aspect Ratio

Aspect ratio is the ratio of a tire's width to it's height. A 60 series tire, therefore, is a tire whose height is equal to 60% of its width.

Speed Ratings

All tires carry a speed symbol in the form of a letter indicating the maximum speed for which the tire is intended. The table below shows which speed goes with each letter.

Speed symbol Maximum speed km/h
mph
N 140 87
P 150 93
Q 160 99
R 170 106
S 180 112
T 190 118
H 210 130
V 240 149
W 270 168
Y 300 186
ZR 240 149

Load Indices

The load-index figure imprinted on the sidewall of the tire denotes the maximum load capacity of a tire when driven at maximum speed. A list of load indices and maximum weights is give below:

Li kg Li kg
65 290 94 670
66 300 95 690
67 307 96 710
68 315 97 730
69 325 98 750
70 335 99 775
71 345 100 800
72 355 101 825
73 365 102 850
74 375 103 875
75 387 104 900
76 400 105 925
77 412 106 950
78 425 107 975
79 237 108 1000
80 450 135 1030
81 462 110 1060
82 475 111 1350
83 487 112 1120
84 500 113 1150
85 515 114 1180
86 530 115 1215
87 545 116 1250
88 560 117 1285
89 580 118 1320
90 600 119 1360
91 615
 
92 630
 
93 650

Tire Age

Tires carry a three digit age code on the sidewall indicating the month and year of manufacture. For example 129 means the tire was manufactured in December 1999.

Other Markings

M+S - Identifies Mud and Snow tires

DOT Codes - Coding satisfying the requirements of the US Department of Transportation contain a mixture of letters and numbers such as DOT DVDE MTA 129

E-Marks - Tires for sale in the European Community must carry an E -Mark in accordance with ECE Reg 30 - eg E4 027550

Air Pressure

The main function of a tire is to carry the load of the car. Therefore, it is vital that correct air pressure is maintained. Over-inflation will cause rapid and irregular wear.
Under-inflation will damage the tire structure leading to possible accidents.
Improper air pressure will not allow the tire to function within its design. For best performance maintain the air pressure recommended by the vehicle manufacturer and/or the tire manufacturer.

Wheel Alignment and Balancing

A major cause of rapid tread wear is improper wheel alignment and balancing. Abnormal noise, vibration and harshness of steering is a signal that alignment and balancing may be improper.

P-Metric and Euro Metric Tire Sizes

P-Metric sized tires always have "P" at the beginning of their size description, such as P225/40 R18, while the Euro Metric tires just leave that out.

Euro Metric tires are the older method for measuring tires, and began by using metric measurements for the tire's width. P-Metric tires were introduced to the United States in the late 70s for passenger car and light trucks, and use an engineering formula to calculate the tire load capacity. This helps car manufacturers design new vehicles with tire standards.

Euro Metric and P-Metric tires are identical in their dimensions and have just small differences in the inflation pressure and load capacity designations. Therefore if two tires have the same measurements, the same speed rating and the same performance category the two are considered the same if used in pairings or sets of four. Just follow your car maker's inflation pressure recommendations.

Performance description

This is a designation at the end of a tire size that combines the load index followed by a single letter referring to the speed rating. Using 25/40 R18 92W as an example, 92W is the Performance description, with:
- 92 as the Load index and
- W as the speed rating.

Load Index
The load indices of tires for passenger cars and light trucks are typically between 70 and 110. An index of 80 can carry 992 pounds, 90 can carry 1323 pounds, and so on.

Speed Rating
This rating is designated by a letter from M up to Z, and applies ONLY to undamaged and properly maintained and inflated tires.
M is for a maximum speed of 81mph.
S & T rated tires are for maximum speeds of 112 and 118 mph respectively, and are often used on tires for family sedans and minivans.
V if for a maximum speed of 140 mph abd can be seen often with Sports sedans and coupes.
W & Y ratings are for more exotic sports cars and carry maximum speeds of 168 and 186 mph respectively.
Z speed ratings is an older designation and means the tires is rated for speeds over 149 mph.

Original Equipment (OE) Tires

Your new car came with tires specifically chosen by the automobile manufacturer. Those tires weren't picked just because they were the right size and a familiar brand. The manufacturer weighed the attributes of various tires and chose the best fit for your car's performance profile.

If you have a high performance car, your tires were chosen for speed and handling. Certain touring cars are built for comfort, and the manufacturer called for tires that provide a smooth ride. Some energy efficient vehicles need special tires designed for less road resistance and better gas mileage.

Tires are designed with various combinations of those and other attributes. When your tires need to be replaced, many people believe it makes sense to replace them with original equipment, to give you the ride and the performance that the automobile manufacturer intended.

Note: When you replace your tires, it is always best to put 4 new tires on your car. It is best to rotate your tires according to the manufacturer's schedule; otherwise, your tires probably won't wear at the same rate. if that's the case, it's okay to replace just the two in the front or the two in the back. For maximum safety, if you replace just two of the tires, it's best to purchase the same tires that are already on your vehicle.

Load Range/ Ply Rating Identifications

"Load range" is a term that is synomymous with and gradually replacing the term "ply rating", and relates to the actual load carrying capacity of the tire. The term refers to a specific tire with its maximum load as used in specific ways. Basically it is an index of tire strength and is expressed as a letter. The higher along the alphabet (A, B, C and up), the higher its load rating will be.

And finally, The Law
  1. Tires on cars, light vans (not exceeding gross weight) and light trailers must have a tread depth of at least 1.6mm across the central three-quarters of the breadth and in a continuous band around the entire circumference of the tire.
  2. Tires must be suitable (ie of the correct specification and size) for the use to which the vehicle is being put and must be inflated to the manufacturers' recommended pressures.
  3. Tires of different types must not be fitted to opposite wheels of the vehicle (for example, radial-ply tires must not be fitted to a wheel on the same axle as wheels already fitted with cross-ply tires and vice versa, and a two-axle vehicle must not have radial ply tires on the front axle if cross ply tires are fitted to the rear axle)
  4. No tire must have a break in its fabric or a cut deep enough to reach the body cords. No cut must be more than 25mm or 10% of the tye's section width in length, whichever is greater.
  5. There must be no lump, bulge or tear caused by separation or partial fracture of its structure, neither must any portion of the ply or cord structure be exposed.

© 1999-2017 by Gigatires Tires LLC / tires-direct.com 09/21/2017 12:07