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Reading A Tire

Do the markings on the sidewall of your tire look alien to you? Are you confused to decide what tire suits your vehicle? If yes, then you have come to the right place. We have got you covered with all the information needed to understand the numbers and ratings that are stamped on the tires. This information is the key to getting out of the turmoil of tire specifications

Tire Size Specifications

The tire size helps you determine what size you must buy for the new tires. Going for new tires with the same tire specification is generally the best idea as it eliminates any guesswork that goes into upgrading tires. Simply remove the old ones and swap them out for new ones. The trick here is to focus on three primary numbers, for example, a tire with P205/55R16 90H - the three primary numbers will be 205, 55, and 16. 205 tells you how wide the tire tread is in millimeters. 55 refers to the aspect ratio of tire sidewall which is a representation of the sidewall height as a factor of the width of the tire. So the actual sidewall height here will be 113 mm (~4.5 inches). Refer to this guide to learn more about how to calculate tire size in mm and inches. Finally, 16 indicates the wheel or rim diameter in inches. 

The ‘P’ in the specs refers to passenger tires according to the p-metric standards. Some tires have no letter preceding the number, which also means passenger tire. While an ‘LT’ prefix refers to light trucks for truck and SUV tires. The ‘R’ after the aspect ratio refers to the tire construction. There are basically 3 types of tire constructions, R stands for radial construction (found in modern tires). B means belted bias and D stands for diagonal bias construction. 

When searching for new tires, all you have to do is to opt for the same specifications as the three primary numbers and you will have the right tire size for your wheels. This is the most simplified form of choosing new tires. There are however other factors to be considered like the speed rating and load index.


Tire Load Index And Speed Rating

The numbers and letters that follow the size specifications are the load index and speed rating in order. Essentially, what they mean is how much load each tire is tested to withstand and the maximum speed it was designed to handle. These numbers and letters are derived from a standardized set of values that are showcased in the table

The basic rule here is to stick to the manufacturer’s recommended tire load index and speed rating. These values are usually depicted on the tire placard found on the driver or passenger door jam. Since these are carefully calculated and derived by the manufacturer while designing the vehicle, changing it is not recommended. The speed rating is considered based on the car’s top speed. H means the tire is rated to go 210 km/h (130 mph) which is common to many sedans. Opting for a larger speed rating such as V or W will only cost more and will not be useful unless you have made considerable horsepower upgrades to your vehicle. T and H are common for cars that are often driven on highways. You can stick to S if you do not do many highway runs. 

DOT Codes

The US Department of Transportation or DOT and Transport Canada assigns a set of codes for each tire, which mainly concerns the tire manufacturer except for the manufacturing year. It starts with 2-3-symbols that represent the plant code where the tire was manufactured. Every manufacturer’s plant in every country has a different plant code. The two following symbols show tire size, but only according to the manufacturer and so it does not concern the customers. The next 3-4-symbol code is moulded solely for the manufacturer and is used by them for identification. The last 4 digits are crucial for the customer as it displays the tire manufacturing date. The first pair stand for the week of manufacture and the last pair shows the year. So, for example, the ‘1903’ code translates to 19th week of the year 2003 and ‘03’ is for the year. 

An interesting fact: For years before 2000, this code was only 3 digits long, as it only showed the decade instead of the actual year. 

The date code is very useful as it can help you know how old your tires are and also help you when purchasing new ones. Generally, tires last about 10 years, whether used or stored, and manufacturers recommend swapping them out if they are 6 years or older. When buying new tires make sure they are not more than a couple of years old. 


UTQG Codes

UTQG or Uniform Tire Quality Grade standards are codes that provide estimates of various tire characteristics based on the manufacturer’s tests and can be used to distinguish between different tires that you may be considering buying, to an extent.


Treadwear

This is a 3 digit number starting with 100. A larger number will be more wear-resistant and hence will last longer. Since these tests are conducted differently by each manufacturer, the rating only provides a ballpark estimate for comparison’s sake. 


Traction

This showcases the gripping power of the tire stated as AA, A, B, and C. where AA is the best and C the worst. This score indicates a tire’s wet-stopping ability. 


Temperature

This is also stated in terms of letters - A (best), B and C. This value evaluates the tire’s safe heat dissipativity. A tire that can dissipate heat more effectively will undergo lesser wear and subsequently lasts longer. 

Tire Pressure

The maximum cold tire pressure is mentioned on the sidewall of the tire closest to the wheel, alongside the maximum load capacity. However, this is not the same as the recommended pressure that you have to run the tires with. This number merely shows how much the tire can be safely inflated and does not correlate to the pressure the tires should be used in. You must always stick to the vehicle manufacturer-recommended tire pressure found on the car door sill or in the owner’s manual. 

Tire Ply Composition And Materials

You can also find information on the tire’s internal construction, which is located next to the maximum tire pressure. This data includes the ply composition, the number of layers, and the materials used in the construction. This is mainly displayed for manufacturers and legal reasons and holds no importance to the consumer. 

Severe Service Emblem

Certain tires with winter traction capabilities are graded with an emblem that identifies winter grip and performance. There are two grading systems: ‘M=S’ and 3 peaked mountain/snowflake symbols (3PMSF). These symbols are designated to tires after they pass a certain industry standard winter traction test. The ‘M=S’ symbol is used on tires that perform moderately well in snow, however, the tires are not rated for ice, slush, and cold dry roads. The more modern 3PMSF pictograph certifies tires that match more stringent winter traction tests, allowing them to be used in snow, ice, slush, and even cold dry roads. Dedicated winter tires and some all-weather tires come with the 3PMSF symbol, and the ‘M+S’ symbol is found on some all-season tires. 


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John Framigllia
Technical Writer
Our technical writer is known for simplifying automotive parts and services. Intuitive with various vehicles and manufacturers, he knows how to simplify complicated problems.