As a car owner, you probably have replaced your tires. When buying new tires, the first thing that most of us consider is the recommended tire size. However the tire’s sidewalls contains a whole lot of specifications and safety information. In this article, we are going to guide you on how to read the numbers on tires.
Aside from the size, you may want to consider the speed rating and the load index to achieve optimum performance, safety, and fuel economy. Moreover, the manufactured date, materials used, maximum permissible pressure are some of the other information you can find in the sidewall of your tires. And we will help you decipher each alphanumeric code.
How to read the numbers on tires: Tire Size or specification
One of the basic tire information a car owner should know is the tire size. It is the alphanumeric code imprinted on your tires, usually after the tire name. It is also called tire specification. But the tire specs does not end with the physical dimension of the tire alone, it contains a whole lot of information. We are going to use the tire below as an example.
The first character in your size or specification is a letter to indicate the tire class. We have the letter P in our example, which means P-metric or commonly called as a passenger class. It is a tire class for lightweight vehicles such as domestic cars, CUVs, and minivans. It is also called the Passenger class. The other tire classes are as follows:
LT – The letters denote light trucks. It is a tire class for heavier vehicles such as SUVs, vans, pickup trucks.
ST – The letters mean special trailer service. These are tire classes for much heavier trucks, including heavy trucks and trailers.
C – This tire class is less common. It means commercial which were designed for commercial vans and delivery trucks.
T – It stands for temporary. It is a spare tire that should only be used in the case of a flat tire.
We have barely scratched the surface on how to read the numbers on tires. The next set of code is something that most of us are familiar with – the dimensions.
Tire dimensions or measurements
Dimension is a measurable aspect of an object, such as length, depth, or height. Depending on where you live, dimensions are either measured by the imperial system or the metric system. For some reason, tire manufacturers decided to mixed things up to determine their dimensions.
After the tire class (P), the first number (205) is the width of the tire/tread in millimeters (mm).
The second number (55), followed by the slash, is the aspect ratio. It measures the sidewall in mm from the top of the wheel to the top of the tire. You can calculate this number by dividing the height by width. So, if a tire has an aspect ratio of 55, meaning its height is 55% of its width.
Next, you will encounter the letter R. It pertains to the internal construction of the tire. Letter R means radial construction. Other examples are B for Bias-ply or D for Diagonal.
Finally, the last number (19) is the diameter of the wheel in inches. So basically, the tire’s dimension uses, mm, % and inches to measure its size.
Why do tire manufacturers use not imperial and metric systems?
Wheel size has always been measured in inches. The first manufacturer to market radial-ply construction, the most common tires on the road today, was Michelin. Radial width uses metric or millimeters, whereas, wheel diameters remained using imperial to maintain interchangeability between tire types.
The number after the wheel diameter denotes the load index. In our example, 92 is a code for the maximum load it can carry when fully inflated. The higher the number the higher the capacity.
In our example, the load index of 92 can carry 1,389 lbs. when fully inflated. Multiply this number by four (the number of tires), your car’s maximum load capacity would be 5,556 lbs. Passenger tires have a load index ranging from 75 to 105.
After the load index, is the letter H, which denotes its speed index. The speed index is also an important factor when choosing a tire. It is the fastest speed your tire can handle before it no longer operates as intended.
The letter H in our example means the speed index is up to 130 miles per hour. However, this should not translate to the maximum speed you can drive, which is heavily impacted by, weather or road conditions.
Depending on the manufacturer, your tire may or may not contain this letter. Load range defines the maximum allowable air pressure and how much weight it can carry. Sometimes it is called the ply rating. In our example, the two-letter following the speed is XL. It means extra load or sometimes RL (reinforced load). Other load range code includes:
SL — Standard load. If there is no code, we can still assume it is a standard load
LL — Light load
Trucks use B, C, D, E, or F, which have different corresponding ply ratings, ie., 4-ply, 6-ply, 8-ply, and so on.
How to read numbers on tires – DOT tire identification number
Aside from the tire specifications, there are still additional markings in your tire that can be useful.
In the example above, the letter DOT means Department of Transportation. This is followed by an alphanumeric code (AF WD9E), which indicates manufacturing information. The manufacturing code varies from manufacture as they have their codes such as location and batch numbers in case of a product recall.
However, the last four digits are the date of manufacture which is standard in all tires. Finally, the first two numbers identify the week, and the last two numbers identify the year of manufacture. In the example above, 24 indicates the tire was made in the 24th week of the year. The 15 number indicates it was made in 2015.
What does M+S mean on a tire?
These are all-season tires for muddy and snowy conditions. Depending on the brand, it may be shown as M&S or just MS. It is probably the last thing you need to know on how to read the numbers on tires.
Key takeaways on how to read the number on tires
Before you read the article, you probably know the size or dimension of your tires. But as we have discussed in this article, there is a whole lot of information to be learned just by looking at the sidewall of your tire.
In this guide, you learned how to read the numbers on tires. The sequence of numbers detailing the load index, speed rating, tire size, construction, can help you choose the right tires for your vehicle.