Tire Sidewall Markings Explained
Monday, June 15th, 2020
How to Read the Markings on your Tire Sidewall
The markings found on a tire’s sidewall contain important information regarding size, proper operation of your tire, and other safety information. Many of us don’t take time to read it all, and it may be a bit confusing if we don’t know what those numbers and markings mean. Below is a quick guide to understanding what those markings mean and why they’re important.
Sidewall of a tire with tire size showing.
Tire Application - Or what the tire was designed for
When needing to find a replacement tire the first thing we seek out is the tire size. But first, we need to confirm we are searching for the right type of tire.
Most tire sizes will begin with a letter(s) to indicate what type of vehicle or application they are designed for. Like in the examples below.
P-Metric, Metric, and Euro-metric
P215/75R15
“P” - When “P” is found at the beginning of tire size it indicated that the tire was designed for fitment on a passenger vehicle such as cars, minivans, SUV/CUVs. In some instances, pickup trucks that were not meant to carry heavy loads and are mainly run on the highway will have original tires that are designated as P-Metric. These types of tires generally provide a smooth ride, good handling, and are known for having a long tread life.
Metric or Euro-metric sized tires do not have a “P” at the beginning. These are also installed on passenger vehicles. The main difference between the two is the load-carrying capacity of each. Generally, the metric or Euro-Metric sizes tires have a greater load index and load-carrying capacity.
LT- Light Truck Tires
“LT” - When LT is found at the beginning of a size it indicates the tires are constructed differently than P-Metric tires and are meant for vehicles that are capable of carrying heavier loads, towing, or designed to be driven off paved roads and onto gravel or even harsher terrains. These types of vehicles include larger trucks, SUVs, and vans.
“LT” may also be found at the end of a tire's size. This sizing convention is typically known as a “flotation size”. These tires are often wider providing a larger contact patch and are specifically designed to “float” over loose dirt/sand.
Flotation sizing also differs from metric as the tire’s dimensions are described inches, rather than millimeters.
ST- Special Trailer
When ST is found before a tire size it indicates the tire is for “Special Trailer” use only. The types of tires are found on travel trailers, fifth wheels, horse, boat, and other utility trailers.
ST tires are designed for use on trailer axle positions and not intended to be installed for steering or drive axle positions. They are constructed to meet the higher load carry capacity of trailer applications.
Tire Size
Now that we're familiar with the different tire types, let's take a deeper look at how to read tire sizing and what information the additional markings provide.
Section Width
In metric sizes, the first set up numbers indicate the section width of a tire. The tire size above measures 275 mm at the widest point of the outer sidewall to the widest point in the inner sidewall. Note: Measurements are taken once a tire has been mounted on the recommended wheel width and allowed to sit for 72 hours.
Many drivers prefer to convert the mm to inches to better understand tire measurements.
25.4 mm = 1 inch. You can use an online converter to help convert millimeters to inches.
In flotation sizes, the section width is expressed by the middle number.
35x12.50R17. In this example, the tire width is 12.5 inches.
Aspect Ratio
There is a forward slash following the section width measurement of a tire. This second number is the tire’s aspect ratio, otherwise known as the sidewall height. This measurement refers to the distance from the wheel rim to the top of the tire’s tread. The larger the aspect ratio number is, the taller the tire’s sidewall is. This is often referred to as the tire profile, as in low profile tires.
Ply Rating or Load Range
Ply rating goes back to when bias tires were actually constructed of layer upon layer of cotton fabric plies to give the tire more strength. Therefore, the ply rating referred to how many plies had been used in the tire's construction.
Today, tire plies are made of a stronger material that requires fewer actual plies. So when we say "ply-rating", we are referring to the equivalent strength of a tire compared to earlier cotton fabric ply construction.
For example, you may see a tire sidewall market with "2 PLIES / 6-PLY RATED" as on our Ground Buster III XC knobby style tire for ATVs.
This means the tire is made with two actual plies but has the strength of a 6-ply tire.
Speed Rating
Speed Ratings are based on the fastest speed a tire can safety achieve and still perform as specified. A tires speed rating is based on lab settings with new and undamaged tires under controlled conditions. The letters A-Z correspond to a specific speed which can be easily referenced on the speed rating table.