Tires are probably the part of your car that you don’t usually take notice of unless it goes flat or blown out. It could serve you for years without any problem. Checking on the tire pressure from time to time is one of the things you can do to extend its life but let’s face it, time will eventually come that you will have to buy a new set of tires. It can be very tiresome and expensive on your end but remember that tires play a very critical role in your car and in your daily driving.
The tire is the only part of the car that has a direct contact on the road. Things like the grip, tread, and tire wear should be taken into consideration as it can affect the overall handling and performance of your car, and even the safety of its driver and passengers.
There are lots of factors involved when buying a new tire. You need to consider your driving style, budget, car load, and even where you live. You also have to choose the right size, width, and treading capacity of your tire. These details and more can be found in the letters and numbers that are imprinted on the sidewall of your tire. It is usually imprinted in a code or series such as P215/65R15 89H or 215/65R15 89H. These series can be very confusing to read and understand. Let us decode it and understand the meaning of each and every character.
The Tire Code
The tire code provides all the information needed to make sure that you will be purchasing the right tire for your car and your needs. It provides the service type, tire width, aspect ratio, internal construction, wheel diameter, and the service description like the load index and speed rating.
- Service Type –The service type determines the type of vehicle the tire is designed for. You have to know what type of tire you need to get according for its intended use. You can find it in the first letter of the series code. Here are the common Service Types you can see imprinted on your tire sidewall.
P – This stands for passenger vehicles. This includes regular cars, SUVs, crossovers, minivans, and small pick-up trucks with a load capacity of ¼ to ½ ton.
LT – This means light truck vehicles. This includes some SUVs, medium vehicles and pick-up trucks with a load capacity of ¾ to 1 ton. Sometimes, LT can also be found as the suffix of the code. This has a very different meaning from the LT found at the beginning of the code. The suffix LT means the tire is for heavy-load vehicles.
T – This stands for temporary tire. This is used as a spare tire or a replacement while the main tire is still under repair.
ST – This means a special trailer tire. It is used for boat trailers, travel trailers, and other utility trailers.
C – This is in Euro-metric sizes that can be usually seen at the end of the size description. C stands for commercial. It is usually used for delivery vans and trucks.
M – M is for motorcycles
No Letter – It is also in metric sizes and commonly known as the European size. It is measured in millimetres and it is mostly for vans and SUVs in the US.
- Tire Width – The tire width or the section width is found after the Service Type letter. It is the first three numbers of the code. The numbers are in millimetres, measuring the distance of the outer sidewall to the inner sidewall.
- Aspect Ratio – This is the height to weight percentage ratio. The forward slash (/) separates the tire width from the aspect ratio. The two-digit numbers after the forward slash is in percentage. It is the height of the inflated tire sidewall measured from the wheel rim to the top of the tread. Basically, it is the tire sidewall height divided by the tire width. The tires with lower aspect ratio give better handling and cornering while the higher aspect ratio tires give cushioned and quiet ride. There are rare instances that the tires have a three-digit number aspect ratio. This is read in millimetres instead of percentage.
- Internal Construction – This is the letter after the two- or three-digit number aspect ratio. This letter determines the internal construction of your tire – how the layers on your tire are built. There are three common types of internal construction. There are also some imported tires that have a letter F after the internal construction letter on the code. This means that the tire has a European Tyre and Rim Technical Organization or ETRTO-adopted self-supporting run-flat construction.
R – This means Radial construction. This is commonly used now, the most modern type of internal construction. This type has its plies – which are made of layers of strong cords composed of a blend of polyester, steel and fabric and coated with rubber – arranged at 90 degrees at the direction of travel or radially to the circumference. This type of tire is also being built with steel belts to strengthen it more.
D – This is for diagonal construction or bias-ply. The plies in this construction are layered diagonally in criss-crossed patterns. The cords or plies run at a 45-degree from bead to bead. This is the oldest tire construction ever used. Its fixed structure with a narrow tread and sharp shoulder give out a more vintage look. The diagonal construction tires are still being used now by some trailers and motorcycles.
B – This means Bias-belt construction. This is similar to the bias-ply construction but with extra steel belt layers beneath the tread creating a more reinforced structure. The structure provides an additional support and a more firm inner material to bond with the tread.
- Wheel Diameter – This is the two-digit number that can be seen after the Internal Construction letter on the tire code. This is the diameter of the wheel in inches measured by the distance between the two bead seat areas of the rim onto which the wheels can be mounted. The wheel diameters that are commonly used are 8”, 10”, 12”, 13”, 14”, 15”, 17”, 18”, 19”, 20”, 22”, 23”, 24”, 26”, and 28”. There are some tires with irregular diameters such as 14.5”, 16.5”, and 19.5” that are mostly used for heavy-duty light trucks, trailers, and cube vans. These types of tires are not to be used or mixed with the regular-sized tires.
- Load Index – This is the two- or three-digit figure that you can see after the gap. It specifies the load index or how much weight a properly inflated tire can support. The load index only measures the capacity of a single tire. If you want to know the load capacity of the whole set, just multiply the capacity by four. You can refer to the chart below to determine the load capacity of each tire load index.
|Load Index||Load Capacity (lbs.)||Load Index||Load Capacity (lbs.)||Load Index||Load Capacity (lbs.)|
- Speed Rating – The last figure on the code is the speed rating. It is represented by a letter or two or sometimes a combination of a letter and a number. It specifies the maximum speed it is safe to travel at a certain period. The tires dissipate heat and the higher the speed rating is the better it can handle the heat and give a better control or handling at faster speeds.
This is not a recommended driving speed. Always make sure to follow the speed limits on the road and highways. If you have a temporary or a replacement tire, it must have the same speed rating as your main tires to ensure that your vehicle’s speed capability is maintained. If your vehicle has tires with different speed ratings, the maximum speed capability of your car is no more than the lowest speed rating of all tires. Refer to the chart below to know the speed rating code and its speed capability.
|Speed Symbol||Speed (mph)|
*The tires with a speed capability of more than 149mph, a ZR might be seen in the size designation and the speed of 186mph and above will have a ZR with a Y speed symbol in brackets.
The tires with a speed rating of W, Y, and (Y) can endure the highest speeds. It is usually used in rare sports cars or any car that runs 168 to 186 miles per hour and above.
For the people living in the US, the DOT or the Department of Transportation code must be imprinted on all the tires that are marketed and used there as it is a requirement of Federal Law. While this code is for the manufacturers, drivers can also learn what it stands for. On the sidewall of the tire, you can see the letters DOT imprinted on it, followed by a Tire Identification Number or TIN that includes the Plant Code. The Plant Code is a two- or three-digit number or a combination of letters and numbers that provide the code of the manufacturer of the tires. You can search online for the list of plant codes and what manufacturer each code represents. This is also a way for you to know where your tire was manufactured. Next is the Tire Size, then the Optional Code that is usually used by the manufacturers to provide information of the tire category, tread or patterns. They usually use it to make recalling defective tires easier. The last four digits of the TIN is the Date Code. The first two numbers specify the week and the last two numbers specify the year it was made. For example, 4319 means that the tire was made during the 43rd week of the year 2019. It is useful information since tires usually deteriorate over time. The newer the date the better the tire is.
Knowing and understanding the numbers on your tire is very important as it can help you choose the right tire for your vehicle. The tire code is not something you have to decode using complicated formulas. In fact, it is the easiest way for you to know the basic things or information about your tire that you need to consider before making a purchase.
The right size of the tire can fit into your rim perfectly allowing no room for errors or damages. The right load index and speed rating can help your car be in its perfect driving condition. The dates on the tire sidewall can help you choose a tire from a fresher batch or if the tire that you are already using still has a long way to go before you can retire it. The numbers or figures on your tires definitely matter and can provide a great help.