How to Read the Sidewall of a Tire
Reading the marks on the sidewall of a tire is a language all on its own. While you may know your 2020 Jeep Wrangler has P255/70R16 104S tires, perhaps you've wondered what exactly that means. In this article, we will go over the basics of what each mark means, along with taking a closer look.
Note: Most vehicles have several types of tires that will work for them. At Saunders Motors, we have a team of knowledgeable parts and service staff who can help you find the right tire for your vehicle. Click here to schedule an appointment with an advisor today.
First, let's take a look at a simple breakdown of the markings on the sidewall, using the Wrangler example from above:
Now that we understand what each number and letter refer to, we can get into what they actually mean:
Tire Type/ Class:
The tire type, simply put, tells you what kind of vehicle the tire is made for. There are three different classes;
- P - This stands for "passenger vehicle", which includes cars, SUVs, crossovers, minivans, and certain small trucks.
- LT - This stands for "light truck", which is for vehicles that tow trailers or have a load capacity of 3/4 -1 ton.
- ST - This stands for "special trailer" and is for on trailers, including fifth wheel, boat trailers, travel trailers, and utility trailers.
Note: If there is no letter before the first number, then it is likely a European tire, and therefore follows a different marking system.
The tire width is measured from the inner sidewall of the tire across to the outer sidewall. It is typically measured in millimeters, and may also be referred to as the "section width".
The aspect ratio is a two-digit number separated from the tire width by a forward slash mark. The larger the aspect ratio number, the higher/taller the tires' sidewall will be. Sometimes it is also referred to as the "profile".
Since the aspect ratio is expressed as a percentage, a small calculation is required if you wish to know the sidewall height of your tires.
Important Note: For this next step, you must convert the tire width from before from millimeters to inches, and express the ratio as a decimal point.
Simply multiply the tire width (in inches) by the aspect ratio (as a decimal point) to get the sidewall height in inches. In the above example, this would look like:
The construction type refers to how the inside of the tire is structured. There are two different types of structures:
- R - this indicates the tire has a radial internal structure, the modern industry standard. These tires have an improved ability to grip the road, give better gas milage because of their lower rolling resistance, have increased durability, and offer a smoother ride. Inside the tire, the plies (layers of strong cords consisting of a blend of polyester, steel and fabric coated with rubber) are positioned perpendicular to the direction of travel.
- D - this indicates that the tire has a bias-constructed internal structure. They differ from radial tires as their plies are positioned in a criss-crossed layout. These tires are also commonly referred to as "conventional", "x-ply", or "cross-ply", and are mostly seen on certain trailers and motorcycles.
Note: Some run-flat tires are identified with an F followed by the type of internal construct.
The wheel diameter is measured from one side of the rim straight across the centre to the other side of the rim. The wheel diameter is expressed in inches.
The load index indicates how much weight the tire can hold. To find the maximum load weight, find the load index number on the chart below. The corresponding weight number is the maximum load weight.
The speed rating indicates the top speed it is safe to travel at with the tire. Similar to the load index, the speed rating is also located using a chart.
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