6 Things You Need to Learn about the Numbers and Letters on Your Tires

Image source: continentaltire.com

Aside from your car’s fuel, the right size of the wheels also plays a key role in keeping your vehicle in tip-top shape. If you have noticed, aside from the brand of your tires there are other markings on it, particularly letters and numbers. What do tire size numbers mean? And what do these letters on my tires mean?

These numbers and letters tell a lot of information about the tires’ purpose, its dimensions, the load capacity it could hold, and its durability in high temperature and high speed.

Tires make your car stop or go. Tires make a lot of difference when you find yourself in a difficult situation where you need to steer clear away from danger or from harming others. It’s like a piece of clothing, which you need to make sure fits perfectly.

Understanding the letters and numbers on your tires is important because it will help you determine which type of tire to purchase as a replacement. You need to get the proper size of the tires and make sure that all tires are marked the same.

What do the letters on tires mean?

The easiest thing to point out in your tire’s sidewall would be the brand of the tire you are using. The first letter or letters on the markings will indicate the type of the tire. Under the brand name we usually see the combination of letters and numbers starts with letters, like the following examples:

  • P225/45R17 91V
  • LT235/75R15 104/101S/C
  • T145/70R17 106M

These numbers show a lot of information, including the six things that you need to know about the markings on the tires:

  • Type of the tire
  • Width
  • The height of its sidewall also called aspect ratio
  • Construction
  • Size of the wheel where it fits
  • Load capacity
  • Speed rating

You can also learn about the production date of the tires, as well as maximum inflation rating, the Uniform Tire Quality Grading (UTQG), and its temperature and traction ratings. Let’s get to learn more about these markings. Let’s start with the letters at the beginning of the combinations.

 

1.Type of tire 

What do LT and P mean on tires? The first letter on a tire shows the tire’s class. P means the tire is used for a passenger vehicle. Passenger vehicles include cars, light-duty pickup trucks, and minivans.

Image source: drivinglife.net

LT, on the other hand, indicates that the tires are made for light trucks, which require higher inflation pressure than that of P tires. This type of vehicle includes full-sized vans, SUVs, and some of the larger pickup trucks with a capacity of 3/4 ton to 1-ton.

How about the letter T? If you see a combination of tire size beginning in T, it indicates that the tire is a temporary spare. These tires are used only for a short period, usually during an emergency. They should be replaced once the vehicle is repaired.

What do ST and C mean? The ST combination on tires indicates that the tires are special trailer tires that are used on utility trailers, cars or boats. C, on the other hand, can be found in the middle part of Euro-metric tire sizes. The letter C means the tires are commercial tires used for delivery trucks or vans that can carry heavy loads.

If there is no letter before the set of numbers, the tire indicates a Euro-metric tire. The important thing here is that P-Metric tires have different load capacities compared to Euro-metric tires. The recommended tire size and inflation pressure for the vehicle are detailed in the owner’s manual, however. Tire companies are also proficient in determining which rubber is right for you.

2.Width

The tire size first number shows the width of the tire as measured at its widest part in millimeters, from one sidewall to the other. So for this marking: P225/45R17 91V, the tire intended for a passenger vehicle is 225 millimeters wide. Cars that are high-performance use tires with higher section width than other vehicles.

Aspect ratio

The next two numbers after the width of the tire show the sidewall height in the percentage of the tire’s width.

So if the tire markings indicate P225/45R17 91V, it means that the height of the sidewall is 225 multiplied by 0.55 or 101.25 millimeters. The number 45 means that the height is equal to 45 percent of the width. Bigger tire ratio indicates bigger and taller tire sidewall. Tires with higher aspect ratios also mean that the passenger vehicles have smoother rides.

Lower tire ratio, on the other hand, would mean that the car is of low-profile similar to high-performance cars.

Construction

What does R mean on tires? After the aspect ratio in this example, P225/45R17 91V, you can find the letter R which indicates that the tire is a radial tire.

Radial tires, also called radial-ply tires, have layers of fabric like polyester and/or nylon plies that run across the circumference of the tires. Its tread is made stronger by the additional layers around it and it is the most common type of vehicle tires.

Aside from the radial type construction on a tire, you may also find letter B or D instead of R. B stands for bias-belted and D stands for diagonal bias construction.

Bias-belted is similar to the original bias tire (D) construction. Bias tires have a series of tire plies, such as nylon or steel, under the tread to support the vehicle and keep its form when filled with air. The plies offer internal support and are layered diagonally.

However, with bias-belted tires, stabilizer belts, made of steel or other corded material, are placed at the plies’ different angles above. It offers more support and a smoother ride. These tires are preferred by those who drive classic cars. It is also used in some light trucks like pickup trucks and SUVs.

Here are some of the pros and cons of using radial tires:

Advantages:

  • Radial tires have flexible sidewalls.
  • It saves fuel consumption because of lesser rolling resistance.
  • It provides less vibration.
  • It extends the life of tires due to lesser heat generated.

Disadvantages:

  • Radial tires have low lateral stiffness causing more tire sway as you speed up the vehicle.
  • Its sidewall tends to bulge making it prone to damage and puncture.  

3. Size of the wheel

After the letter R, a number that indicates the wheel size where it fits. So for the example, P225/45R17 91V, R17 means the tire is made of radial-ply construction and that it fits a 17-inch rim. It’s a common knowledge for car owners that tires only fit a wheel that has an identical inner diameter with it.

The common widths of tires, measured in inches, are: 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28. However, there are also “inch rim” sizes, or the measurement of a rim diameter in inches, that are made in half-inch diameter.

These unique wheel diameters are usually used in some heavy-duty light trucks and trailers. The sizes of the tires may come in 14.5, 15.5, 16.5, 17.5, and 19.5. Vehicle owners should always keep in mind that rim diameters with half-inch measurements should not be used with traditional inch rim tires and wheels. Keep in mind that tires and wheel diameters should always match.

4. Load capacity

The remaining numbers in the tire combinations indicate the load and the speed rating of the tire. Let’s use the same example: P225/45R17 91V, R17. The markings 91V are the load index and the speed rating, or the service description.

The load index (91) is the assigned numerical value for the tire size to compare relative load carrying capabilities. The table below shows the load indexes of the tires that are commonly used in passenger cars and light trucks. Using this table, we can compute that a vehicle with a 91 load index can carry 615 kilograms or 1,356 pounds.

Load IndexPoundsKilograms Load IndexPoundsKilograms Load IndexPoundsKilograms
7073933589127958010822051000
7176134590132360010922711030
7278335591135661511023371060
7380536592138963011124031090
7482737593143365011224691120
7585338794147767011325351150
7688240095152169011426011180
7790841296156571011526791215
7893742597160973011627561250
7996343798165375011728331285
8099245099170977511829101320
811019462100176480011929981360
821047475101181982512030861400
831074487102187485012131971450
841102500103192987512233071500
851135515104198490012334171550
861168530105203992512435271600
871201545106209495012536381650
881235560107214997512637481700

(Table: Load Index Chart)

What do SL and XL mean on tires? The markings of XL on tires sometimes confuse customers who think that XL stands for an “extra large” tire. But the XL on tires stands for extra load and it indicates that these tires can use higher air pressure to support more weight compared to standard tires that have the same size.

On the other hand, SL means standard load. Tires with standard load can have a maximum inflation pressure of 35 psi, while tires with extra load can be carried at a maximum inflation pressure of 41 psi. To check whether your current tires have a capacity for extra load, look at the sidewall. The XL tires will have a higher load index than a standard tire. An XL tire may have a bit stiffer sidewall than those with a standard load.

Keep in mind that inflating your tires to maximum pressures can cause uneven tire wear and can compromise your safety.

RatingMaximum Speed
Q100 MPH
S112 MPH
T118 MPH
U124 MPH
H130 MPH
V149 MPH
W168 MPH
Y186 MPH
ZOver 149 MPH

5. Speed rating

Meanwhile, the letter V that follows the load index shows the speed rating of the vehicle. Speed rating corresponds to the maximum speed that a tire can sustain when it is carrying the recommended load capacity. The rating system uses letters from A to Z. Here, you can see that a tire with a 91V mark can them run at a maximum speed of 149 mph while carrying 615 kilograms or 1,356 pounds load.

What does ZR in tire size mean? ZR or Z-rated markings mean the tires can go over 149 mph in speed. Car manufacturers used ZR to refer to high-performance tires before 1990. Later on, the ratings got refined to V-, W- and Y-speeds.

Tire’s age and the DOT serial number

Image source: www.discounttire.com

Aside from the six main meanings of the letters and markings on the tires, you can also find the Department of Transportation (DOT) serial number on the inner sidewall near the rim.  

The serial number shows the year 2000, followed by the four-digit production date. The first two digits of the production date indicate the week and the last two digits show year when the tire was manufactured. This will tell you the age of the tire. Before the year 2000, three numbers were used to show the production date.

Uniform Tire Quality Grading

The DOT also required manufacturers to apply a Uniform Tire Quality Upgrading or UTQG to indicate its treadwear rating, temperature range, and traction ability.

This number uses the reference tire with a rating of 100 as its baseline. So if a tire with a rating of 240 would probably last 2.4 times longer than the reference tire. The higher the number, the longer the tire is expected to last.