How to Read Tire Size: A Complete Guide to Tire Size Numbers and Letters
Last updated on July 24th, 2026
Looking at the sidewall of a tire can feel confusing at first. You might see something like P225/65R17 102H and wonder what all those numbers and letters mean.
The good news is that tire size markings follow a simple pattern. Once you understand what each part represents, you can quickly read a tire size, understand its basic specifications, and compare different tire sizes.
In this guide, you’ll learn how to read tire size numbers and letters, where to find the correct tire size for your vehicle, what terms such as P, LT, XL, load index, and speed rating mean, and how common tire size formats differ.
We’ll also look at how to calculate tire sidewall height and overall diameter and what to consider if you’re thinking about changing your vehicle’s tire size.
Where to Find Your Vehicle’s Tire Size
Before buying replacement tires, you need to know the correct size for your vehicle. There are several places where you can find this information, but they are not all equally useful.
Driver-Side Door Jamb
The driver’s side door jamb or door placard is usually the best place to start.
Open the driver’s door and look for a label attached to the door, door frame, or nearby body panel. Depending on the vehicle, the label may list the original tire size, recommended tire pressure, vehicle weight information, and other specifications.
The tire size shown on this label represents the manufacturer’s recommended fitment for your vehicle. It is generally a better reference for choosing replacement tires than simply copying the size printed on your current tires.
Owner’s Manual
Your owner’s manual is another reliable source of tire information. It may list the recommended tire size for your specific vehicle and may also include information about different tire sizes available for different trims or driving conditions.
If your vehicle has more than one approved tire size, the owner’s manual can help you understand which sizes are appropriate.
Tire Sidewall
You can also find the tire size printed directly on the sidewall of the tire. It is usually displayed in large lettering, such as:
P225/65R17 102H
This tells you the size and several other specifications of the tire currently installed on your vehicle.
However, keep in mind that the current tire may not be the original size recommended by the manufacturer. A previous owner may have changed the tires or wheels. For that reason, check the driver’s door placard or owner’s manual before purchasing a new set if you are unsure.
How to Read a Tire Size: P225/65R17 102H
Tire sizes may look like a random mix of numbers and letters, but each part has a specific meaning. For example, take the tire size P225/65R17 102H.
You can read it from left to right:
| Marking | Meaning |
| P | Passenger tire |
| 225 | Tire width in millimeters |
| 65 | Aspect ratio |
| R | Radial construction |
| 17 | Wheel diameter in inches |
| 102 | Load index |
| H | Speed rating |
P — Passenger Tire
The P means the tire is designed for passenger vehicles. You commonly see this marking on cars, minivans, and some SUVs and crossovers.
Not every tire has a letter at the beginning. For example, a tire may simply be marked 225/65R17. The absence of a P does not automatically mean the tire is unsuitable for a passenger vehicle.
225 — Tire Width
The 225 is the tire’s width, measured in millimeters from sidewall to sidewall.
So, a 225 tire is approximately 225 mm wide. The actual measured width can vary slightly depending on the tire brand, model, and wheel width.
65 — Aspect Ratio
The 65 is the tire’s aspect ratio. It means the sidewall height is about 65% of the tire’s width.
In this example, the sidewall is approximately 65% as tall as the 225 mm tire width. A lower aspect ratio generally means a shorter sidewall, while a higher ratio means a taller sidewall.
R — Radial Construction
The R stands for radial. It describes how the tire’s internal layers are arranged.
Radial tires are the standard type used on most modern passenger vehicles, so R is very common in today’s tire sizes.
17 — Wheel Diameter
The 17 means the tire is designed to fit a 17-inch wheel.
This number refers to the wheel diameter, not the tire’s total diameter. A tire marked R17 must be mounted on a 17-inch wheel.
102 — Load Index
The 102 is the tire’s load index. It is a standardized number that corresponds to the maximum weight the tire can carry under specified conditions.
The number itself is not the weight in pounds or kilograms. You need a load index chart to find the exact weight capacity associated with 102.
When replacing tires, make sure the new tires meet or exceed the load capacity recommended for your vehicle.
H — Speed Rating
The H is the tire’s speed rating. It indicates the maximum speed capability of the tire under specific test conditions.
A speed rating is not a recommended driving speed. It is one part of the tire’s overall performance rating, and the replacement tire should meet the vehicle manufacturer’s requirements.
Putting It All Together
P225/65R17 102H describes a passenger tire that is approximately 225 mm wide, has a 65% aspect ratio, uses radial construction, and fits a 17-inch wheel. The 102 identifies its load index, while H identifies its speed rating.
Once you understand this pattern, most standard passenger tire sizes become much easier to read. Always check your vehicle’s door-jamb placard or owner’s manual to confirm the correct tire size, load index, and speed rating before buying replacement tires.
What Does P, LT, and T Mean on a Tire?
Some tire sizes begin with a letter before the numbers. This letter gives you a quick clue about the type of vehicle or use the tire is designed for.
P — Passenger
P stands for Passenger. It is commonly found on tires designed for everyday passenger vehicles, including cars, minivans, crossovers, and some SUVs.
For example, P225/65R17 is a passenger tire size. Not all passenger tires have a P at the beginning, so a missing P does not automatically mean the tire is a different type.
LT — Light Truck
LT stands for Light Truck. These tires are designed for vehicles such as pickup trucks and some heavier-duty SUVs, especially when carrying heavier loads or towing.
For example, LT265/70R17 is an LT tire size. LT tires are generally built for more demanding use, but they may provide a firmer ride than comparable passenger-type tires.
T — Temporary Spare
T stands for Temporary and is used for compact spare tires designed for short-term emergency use.
For example, T125/70D16 may be found on a temporary spare. These tires are not intended to be used as a normal replacement for your regular tires and typically have specific driving and distance limits listed on the tire or vehicle documentation.
The letter at the beginning of a tire size is only one part of the tire’s specifications. Always make sure the tire type, size, load capacity, and other requirements match your vehicle before replacing a tire.
Understanding the Numbers in a Tire Size
The three main numbers in a standard tire size tell you the tire’s width, sidewall height, and wheel size. For example, in 225/65R17, the numbers are 225, 65, and 17.
Understanding these numbers is important when comparing tire sizes or checking whether a different tire may fit your vehicle.
Tire Width
The first number, 225, is the tire’s approximate width in millimeters, measured from one sidewall to the other.
A larger number generally means a wider tire. For example, a 245 tire is generally wider than a 225 tire.
Tire width can affect how a vehicle feels and performs. A wider tire may offer more dry-road grip, but it can also affect steering feel, road noise, fuel economy, and clearance. The actual width can also vary slightly between tire brands and models, even when the marked size is the same.
When changing tire width, don’t look at the width alone. The wheel width, suspension, fender clearance, and vehicle manufacturer’s recommended sizes also need to be considered.
Aspect Ratio
The second number, 65, is the tire’s aspect ratio. It tells you the sidewall height as a percentage of the tire’s width.
In 225/65R17, the sidewall height is approximately 65% of 225 mm, or about 146 mm.
A lower aspect ratio means a shorter sidewall. A higher aspect ratio means a taller sidewall. For example, a 225/55R17 has a shorter sidewall than a 225/65R17, even though both tires have the same 225 mm width and fit a 17-inch wheel.
The aspect ratio affects more than appearance. A taller sidewall generally provides more cushioning over bumps, while a shorter sidewall can give sharper steering response but may transmit more road impact into the vehicle.
Wheel Diameter
The last number, 17, tells you the diameter of the wheel the tire is designed to fit, measured in inches.
In 225/65R17, the tire must be mounted on a 17-inch wheel. A 17-inch tire cannot be safely mounted on an 18-inch or 16-inch wheel.
It is important not to confuse wheel diameter with the tire’s overall diameter. The 17 refers only to the wheel size. The complete tire is larger because it also includes the sidewall above and below the wheel.
When comparing tire sizes, a change in wheel diameter usually requires a corresponding change in aspect ratio to keep the overall tire diameter close to the original. This is why a vehicle can sometimes use different tire and wheel combinations while maintaining a similar overall tire height.
Before changing any of these numbers, check the vehicle manufacturer’s recommended tire sizes and confirm proper fitment. A tire that appears similar on paper may still cause clearance, handling, or speedometer issues if the complete size is not suitable for the vehicle.
Understanding the Letters in a Tire Size
The letters in a tire size provide additional information about how the tire is built and how it is rated for performance. In 225/65R17 102H, the two key letters are R and H.
What Does R Mean?
The R stands for radial. It describes the way the tire’s internal cords are arranged.
Radial construction is the standard design used on most modern passenger vehicles. It helps provide a good balance of ride comfort, handling, durability, and fuel efficiency for everyday driving.
You may occasionally see other letters in older or specialized tire sizes, but R is by far the most common marking on modern road tires.
What Does the Speed Rating Mean?
The final letter, H in 225/65R17 102H, is the tire’s speed rating. It indicates the maximum speed the tire is designed to handle under specific test conditions when properly inflated and carrying the correct load.
Speed ratings use letters rather than directly showing a speed in the tire size. For example, H represents a specific speed capability, while ratings such as T, V, W, and Y represent different capability levels.
A speed rating is not a recommended driving speed. It is a performance rating based on controlled testing. Real-world factors such as tire condition, inflation pressure, vehicle load, weather, and road conditions also affect safe driving.
When replacing tires, choose a speed rating that meets or exceeds the vehicle manufacturer’s recommendation. Using a tire with a lower rating than specified may affect handling and safety, especially during high-speed driving.
The speed rating should also be considered together with the tire’s load index. Both ratings are part of the tire’s overall performance specifications and should match the requirements of your vehicle.
What Is the Tire Load Index?
The load index tells you how much weight a tire can safely carry when it is properly inflated. In 225/65R17 102H, the number 102 is the load index.
The number itself is not the weight in pounds or kilograms. It is a code that connects to a load index chart. In simple terms, the higher the number, the more weight the tire can carry.
This matters because a tire has to match the needs of the vehicle, not just the wheel size. When replacing tires, always choose a load index that meets or exceeds the vehicle manufacturer’s recommendation. A tire with too low a load index may not be suitable, even if the size looks correct.
The load index is one of the easiest numbers to overlook, but it is one of the most important for safe fitment.
What Do XL, SL, and Load Range Mean?
You may see extra markings after the main tire size, such as XL, SL, or a Load Range designation. These markings provide additional information about how much load a tire is designed to carry and how it is constructed.
XL — Extra Load
XL stands for Extra Load. An XL tire has a reinforced construction that allows it to carry a higher load than a standard-load tire of the same basic size when inflated to the appropriate pressure.
You may see a marking such as 225/65R17 102H XL. The XL does not mean the tire is physically larger. It indicates a higher load-carrying version of that tire size.
XL tires can be useful for vehicles that require additional load capacity. However, choosing an XL tire simply because it has a higher capacity is not always necessary. Always follow the vehicle manufacturer’s tire specifications.
SL — Standard Load
SL stands for Standard Load. It identifies a tire built for the standard load capacity associated with its size.
SL tires are common on passenger cars and other vehicles designed for normal everyday use. If your vehicle originally specifies a standard-load tire, there is usually no need to choose an XL tire unless the manufacturer allows or recommends it.
Load Range
Load Range markings are mainly used on light-truck tires, particularly LT tires. They indicate the tire’s load-carrying capability based on its construction and specified inflation pressure.
Common designations include Load Range C, D, and E. In general, a higher load range indicates a tire designed to handle greater loads when used at the appropriate pressure.
Load Range is not the same as the load index. Load index is a standardized number, such as 102, while Load Range uses letters such as C, D, or E. When replacing a light-truck tire, always match the load requirements specified by the vehicle manufacturer rather than choosing a higher rating based on the letter alone.
How to Read Different Tire Size Formats
Not every tire uses the same size format. Passenger cars often use a metric format, while many pickups and light trucks use LT or flotation sizes.
The format may look different, but each part still tells you something about the tire’s size or intended use.
Metric Tire Size: 225/65R17
This is one of the most common tire size formats for passenger cars, crossovers, and SUVs.
225/65R17 breaks down as:
- 225 = approximate tire width in millimeters
- 65 = aspect ratio, meaning the sidewall height is about 65% of the tire’s width
- R = radial construction
- 17 = wheel diameter in inches
The metric format gives you the tire’s width and sidewall proportion rather than directly stating the tire’s overall height.
LT Tire Size: LT265/70R17
LT265/70R17 follows the same basic metric pattern but starts with LT, which means Light Truck.
Here, 265 is the approximate width in millimeters, 70 is the aspect ratio, R means radial construction, and 17 is the wheel diameter in inches.
LT tires are commonly used on pickup trucks and some heavier-duty SUVs. The LT prefix is important because it identifies the tire as a light-truck type rather than a standard passenger tire.
Flotation Tire Size: 33×12.50R17
Flotation sizes are commonly seen on trucks and off-road tires. Unlike metric sizes, they typically give the tire’s approximate overall diameter and width directly in inches.
33×12.50R17 breaks down as:
- 33 = approximate overall tire diameter in inches
- 12.50 = approximate tire width in inches
- R = radial construction
- 17 = wheel diameter in inches
For example, a 33×12.50R17 tire is approximately 33 inches tall and 12.50 inches wide, designed to fit a 17-inch wheel.
Keep in mind that flotation measurements are approximate. The actual dimensions can vary slightly between tire brands and models, so always check the manufacturer’s specifications when exact fitment matters.
The key difference is simple: metric sizes use width and aspect ratio to describe the tire, while flotation sizes state the approximate overall diameter and width directly in inches.
How to Calculate Tire Sidewall Height and Overall Diameter
Knowing how to calculate a tire’s sidewall height and overall diameter helps when comparing tire sizes or considering a different tire size for your vehicle.
Calculating Sidewall Height
Let’s use 225/65R17 as an example.
The 225 is the tire width in millimeters, and 65 is the aspect ratio. To find the approximate sidewall height, multiply the width by the aspect ratio:
225 × 0.65 = 146.25 mm
So, the sidewall is approximately 146 mm (5.76 inches) tall.
This is the height of the sidewall from the edge of the wheel to the outer edge of the tire. Because the calculation is based on the marked tire size, the actual measured height can vary slightly between tire models.
Calculating Overall Tire Diameter
To estimate the tire’s overall diameter, add the height of both sidewalls to the wheel diameter.
For 225/65R17:
Sidewall height: 146.25 mm
Two sidewalls: 146.25 × 2 = 292.5 mm
17-inch wheel: 17 × 25.4 = 431.8 mm
Now add them together:
292.5 + 431.8 = 724.3 mm
Convert that to inches:
724.3 ÷ 25.4 ≈ 28.5 inches
So, the estimated overall diameter of a 225/65R17 tire is approximately 28.5 inches.
This is an estimate based on the tire’s marked size. The actual overall diameter can vary slightly depending on the tire brand, model, tread design, and measuring conditions.
Why Overall Diameter Matters
Overall tire diameter is especially important when changing tire sizes because even a small difference can affect how your vehicle behaves.
- Speedometer accuracy: A larger or smaller tire changes how far the vehicle travels with each wheel rotation, which can make the speedometer read slightly higher or lower than your actual speed.
- Ground clearance: A taller tire can increase ground clearance, while a shorter tire can reduce it.
- Gearing: Changing overall diameter affects the effective gearing of the vehicle. Larger tires can reduce engine RPM at a given road speed but may also make acceleration feel slower.
- Vehicle fitment: A taller or wider tire may contact the fender, suspension, or other components, especially when steering or when the suspension moves.
When comparing a new tire size with your vehicle’s original size, check the overall diameter, not just the tire width. If the diameter changes significantly, you may also need to consider speedometer accuracy, clearance, and other fitment requirements.
For a quick comparison, you can use a tire size calculator to compare the original and proposed tire sizes before making a change.
Can You Change Your Tire Size?
Yes, changing tire size is possible on many vehicles, but it should be done carefully.
Some drivers change tire sizes to improve appearance, increase ground clearance, improve handling, or fit a different wheel-and-tire combination. However, changing size involves more than choosing a tire that looks similar.
You should consider the tire’s:
- Overall diameter
- Width
- Load index
- Speed rating
- Wheel width requirements
- Clearance around suspension and steering components
- Clearance inside the fender
- Compatibility with the vehicle’s systems
You should also consider the vehicle manufacturer’s recommendations.
What Happens When You Install a Larger Tire?
A larger tire can increase ground clearance and change the vehicle’s appearance. Depending on the size change, it can also affect speedometer accuracy, acceleration, gearing, fuel economy, steering feel, and clearance.
A tire that is too large may rub against suspension components, wheel-well liners, or bodywork, particularly when turning or when the suspension is compressed.
What Happens When You Install a Smaller Tire?
A smaller tire can reduce ground clearance and may change the vehicle’s speedometer and odometer readings.
It can also affect gearing and engine RPM. If the overall diameter changes significantly, the vehicle may behave differently from how it was designed to operate.
The goal when changing tire size is not simply to find a tire that fits the wheel. The complete wheel-and-tire combination must be suitable for the vehicle.
You may hear people recommend staying within a certain percentage of the original tire diameter, such as 3%. While maintaining a similar overall diameter is a useful general guideline, there is no universal percentage that guarantees a safe or correct fitment for every vehicle.
A proper fitment check should also consider width, wheel dimensions, offset, load capacity, clearance, and the vehicle manufacturer’s specifications.
Common Mistakes When Reading Tire Size
Reading a tire size is simple once you understand the format, but several common mistakes can lead to buying the wrong tire.
Confusing Tire Size With Tire Pressure
A tire size such as 225/65R17 tells you the dimensions and construction classification of the tire.
It does not tell you how much air pressure you should put in the tire.
The recommended cold tire pressure is normally found on the driver’s door placard or in the owner’s manual. Do not use the maximum pressure printed on the tire sidewall as the vehicle’s recommended inflation pressure.
Confusing Wheel Diameter With Overall Tire Diameter
In 225/65R17, the 17 refers to the wheel diameter.
It does not mean the complete tire is 17 inches tall.
The overall tire diameter includes the wheel diameter plus the height of the tire’s sidewalls.
Ignoring Load Index
Two tires may have the same basic size but different load capabilities.
Always check the required load index when choosing replacement tires, especially for trucks, SUVs, vans, and vehicles that regularly carry heavy loads.
Ignoring Speed Rating
The speed rating is another important part of the tire’s specification.
When replacing tires, make sure the speed rating is appropriate for your vehicle and follows the manufacturer’s requirements.
Assuming the Current Tire Is Automatically the Correct Factory Size
The tires currently installed on a used vehicle may have been changed by a previous owner.
Before buying replacements, check the driver’s door placard or owner’s manual to confirm the manufacturer’s recommended size.
Buying Based Only on Width and Diameter
A tire’s width and wheel diameter are only part of the story.
You also need to consider aspect ratio, construction type, load index, speed rating, tire type, and whether the tire is compatible with your vehicle and wheel.
Conclusion
Reading tire size numbers becomes much easier once you understand what each part of the code represents.
For a tire marked P225/65R17 102H, the P identifies a passenger tire, 225 is the nominal width in millimeters, 65 is the aspect ratio, R means radial construction, and 17 is the wheel diameter in inches. The 102 is the load index, while H is the speed rating.
If you’re replacing your tires, don’t stop at matching the first few numbers. Check the complete tire specification, including the tire type, load index, speed rating, and any required XL or other load classification.
Most importantly, verify the recommended tire size using your vehicle’s driver’s door placard or owner’s manual before buying. If you’re considering a different size, compare the overall diameter and width and confirm that the new combination is suitable for your specific vehicle.
Understanding your tire size helps you make a more informed choice, but correct fitment is about more than numbers alone. The right tire should match your vehicle’s specifications, your driving needs, and the conditions in which you drive.
