Ever needed to shop for new tires only to find yourself staring at some weird numbers and letters that seemingly make no sense? Understanding the letters and numbers on your tires is crucial to ensure you select the right size when it’s time for a replacement.
In this article, we’ll unravel the alphanumeric code on your tires so you’re equipped with all the knowledge to make an informed decision.
How to Read Tire Size
There are 2 ways to measure a tire size: “metric” and “flotation”, also known as “standard”.
The easiest way to quickly see if your tire size is metric or flotation is by looking at the separator character. Metric has a forward slash (/), while flotation has an “X” as a separator character.
Let’s first explain metric tire size as this is the most difficult one to decipher:
Metric
A metric tire size will look something like this: 295/45R15 104S. Let’s break this down to know what these numbers mean:
Tire Type (P):
Some tire sizes will start with a letter P, LT, ST, T or no letter at all. The first letter(s) in the tire size code (if present) indicate(s) the type of vehicle the tire is designed for.
“P” (or no letter at all) stands for Passenger vehicle, implying the tire is suitable for passenger cars, minivans, SUVs, and other light-duty vehicles. This designation is crucial because it sets the tire’s performance characteristics to match the needs of typical passenger vehicles, including handling, load capacity, and ride comfort.
Besides “P”, you might encounter “LT” for Light Truck, which signifies tires designed for vehicles that carry heavy loads or perform towing tasks, like light trucks. These tires typically have higher load ratings and are built tougher than passenger tires.
Another designation you might see is “T” for Temporary, commonly found on spare tires. These are not meant for long-term use and usually have restrictions on speed and mileage.
Finally there is the “ST” designation which stands for ‘Special Trailer’. These tires are developed specifically for use on trailers and campers.
Tire Width (295):
The first three-digit number (“295” in the example below) is the tire width, or the width from sidewall to sidewall measured in millimeters, which in our case translates to 295 millimeters or about 11.6 inches wide.
The width of the tires affects how the vehicle handles and performs. Wider tires have a larger contact surface with the road, offering better grip and handling, which is particularly noticeable during cornering and high-speed maneuvers. However, they can also make the steering feel heavier (without power steering) and may hydroplane more easily in wet conditions due to their larger surface area. Wider tires can also affect fuel consumption negatively due to higher rolling resistance.
Aspect Ratio (45):
The next number (two-digit number “45” in our case), is the aspect ratio which is used to indicate how high the sidewall is. The aspect ratio is the tire’s height compared to its width, expressed as a percentage.
In our example, the tire’s height is 45% of its width of 295 millimeters. This equals to 132.75 millimeters or about 5.23 inches.
The tire ratio plays a significant role in the tire’s performance and the vehicle’s overall appearance. A lower aspect ratio, often seen in performance tires, means a shorter sidewall, which usually translates to better handling and steering response due to reduced sidewall flexing.
Tires with a higher aspect ratio, indicating taller sidewalls, generally offer a smoother ride. The increased sidewall height allows for better absorption of road irregularities, leading to improved ride comfort. However, this can sometimes come at the cost of precise handling.
Construction (R):
The letter “R” in our tire code example stands for radial, which is the type of tire construction.
Radial construction is the most common type of tire construction and refers to the way the tire’s internal ply cords are arranged. Radial tires have ply cords that extend across the tire radially, from one bead to the other, at a 90-degree angle to the direction of travel.
Radial tires are known for their durability, stability, and efficiency. They provide better traction and fuel efficiency, longer tread life, and a smoother ride compared to other types, like bias-ply tires. The flexing of their sidewalls also reduces the rolling resistance, enhancing fuel economy.
Wheel Diameter (15):
The next number on our tire, the number “15”, stands for the wheel size in inches. This number indicates the diameter of the wheel that the tire is designed to fit. In our example, ’15’ means the tire is meant for a 15-inch wheel.
It’s essential to match the tire size precisely to the wheel size for proper fitting and optimal performance.
The wheel diameter is a key factor in determining the overall diameter of the tire, which can affect various aspects of vehicle performance, including speedometer accuracy, acceleration, and handling. Larger wheels generally provide better handling and a more responsive feel, while smaller wheels can offer a more comfortable ride and better traction in adverse conditions.
Note:
To calculate the total height of your tire you need to add the wheel size to 2x the sidewall height. This totals to about 647 millimeters or 25 and a half inches tall.
Load Index (104):
The three-digit number “104” is the load rating, indicating how much weight the tire can carry.
The load index is a numerical code that corresponds to the maximum weight a tire can carry at its maximum safe inflation. On a tire index chart you’ll find the corresponding load capacity of that numerical code. A tire with a load index of 104 for example, can carry 1,984 pounds. It’s crucial to use tires that can support the weight of your vehicle, especially if it’s frequently loaded with passengers or cargo.
Using tires with an appropriate load index is vital for safety and vehicle efficiency. Tires with a load index too low for your vehicle’s weight may wear out faster, offer less stability, and increase the risk of tire failure.
Speed Rating (S):
The last letter indicates the speed rating, the maximum speed at which the tire can safely carry a load under specified conditions. The letter ‘S’ in our example means the tire can safely operate at speeds up to 112 mph.
It’s important to note that this doesn’t imply the vehicle should be driven at these speeds; it’s just an indicator of the tire’s capability.
Speed ratings also give insights into the performance characteristics of the tire. Higher speed ratings typically correspond to tires designed for better handling, cornering, and braking capabilities. They are made of a softer rubber compound for better grip but may wear out faster than tires with lower speed ratings.
Your replacement tires should always have an identical speed rating or a higher speed rating than the tires you have. The slowest speed rating of your tires should indicate the top speed you can drive.
Here are the most common speed ratings:
- Q: 99 mph
- R: 106 mph
- S: 112 mph
- T: 118 mph
- U: 124 mph
- H: 130 mph
- V: 149 mph
- W: 168 mph
- Y: 186 mph
- ZR: 186+ mph
Sometimes on high performance tires, instead of r 15 it will say zr 15. Tires that are rated above 149 miles per hour are allowed to use this, while tires that are rated above 186 miles per hour are required to have this. It is the law.
Flotation
Now that you’ve learned how to decipher “Metric” tire sizes, you’ll appreciate how easy it is to read Flotation (also called “Standard”) tire sizes:
‘30’ is the total height of the tire in inches
‘9.5’ is the tire width in inches, measured from sidewall to sidewall
‘R’ stands for radial
‘15’ is the wheel size in inches
‘LT’ stands for Light Truck,
‘104’ is the load rating
‘S’ indicates the speed rating
For a quick recap of this information, you might want to take a look at our YouTube video
Where to Find the Car Tire Size
If you want to purchase new tires or want to understand your vehicle’s capabilities, it’s important to know where your tire size can be found. These are the primary locations where you can find this information:
- On the Tire Sidewall: The most direct way to find your tire size is to look at the sidewall of your tires. The size is printed in raised letters and numbers.
- Inside the Driver’s Door: Many vehicles have a sticker inside the driver’s door frame, often near the door latch. This sticker not only lists the tire size but also recommended tire pressure and load ratings. Comparing the information on this sticker with your tire sidewall can ensure consistency and verify that the current tires match the manufacturer’s recommendations.
- Vehicle Owner’s Manual: Your vehicle’s owner’s manual contains detailed information about the tire sizes suitable for your car, along with other important tire-related information. Sometimes, the manual lists alternative tire sizes that are also compatible with your vehicle, offering flexibility for different driving needs or conditions.
- Online Vehicle Information: Entering your vehicle’s VIN (Vehicle Identification Number) or model details on a tire manufacturer or tire dealer’s website can often provide specific tire size information. Many tire retailers have online tools where you input your car’s make, model, and year to find the correct tire size.
Meaning of Other Numbers on Tire
DOT and TIN
The Department of Transportation (DOT) mark on tires indicates that the tire complies with all applicable safety standards established by the U.S. DOT. This mark is a certification of the tire’s quality and safety. After the DOT there is a tire identification number (TIN), in our case: BF66 AT11 3918
Here’s what you need to know:
- Location: The series of up to 12 numbers and letters (the TIN), is typically found on the sidewall of the tire, close to the rim. It’s usually preceded by the letters ‘DOT.’
- Code Structure: The code includes a series of letters and numbers, which provide information about the tire’s manufacturer, the plant where the tire was made, the tire size, and the date of manufacture. The last four digits of the TIN code represent the week and year of manufacture. For example, ‘3918’ would indicate the tire was made in the 39th week of 2018. Knowing the tire’s manufacturing date is important for understanding its age, which can be a critical factor in tire safety and performance. Tires degrade over time, even without use.
UTQG
The Uniform Tire Quality Grading System (UTQG) is a standardized grading system established by the U.S. Department of Transportation (DOT). It provides consumers with valuable information about tire treadwear, traction, and temperature resistance.
Here’s what each of these ratings mean:
- Treadwear Grade: This grade provides an estimate of a tire’s longevity or wear rate. The grading is based on a standard test tire, which is assigned a base grade of 100. A tire with a treadwear grade of 200 should last twice as long as the test tire under similar conditions. However, actual tire life can vary based on driving habits, road conditions, and tire maintenance.
- Traction Grade: This grade evaluates a tire’s ability to stop on wet pavement under controlled conditions. It does not indicate traction under icy, snowy, or off-road conditions. Traction grades range from AA (highest) to C (lowest). A higher grade means the tire will offer better wet road traction.
- Temperature Grade: This rating reflects a tire’s ability to withstand and dissipate heat under load. High temperatures can accelerate tire wear and may lead to tire failure. The grades range from A (highest) to C (lowest), with ‘A’ indicating the tire can perform well under higher speed and load conditions.
Where to Find UTQG Ratings
- On the Tire Sidewall: Look for the UTQG ratings near the tire size information. They are usually listed after the size code.
- Online Resources: Tire manufacturers and retailers often list the UTQG ratings in their product descriptions online.
Conclusion
Understanding tire size is more than just a technicality; it’s about ensuring your vehicle’s safety and efficiency. By decoding the numbers and letters on your tire, you equip yourself with the knowledge to make informed choices, ensuring your vehicle is fitted with the right tires for optimal performance and safety.
