Tire sidewalls can look like a string of codes, but the main size marking is easier to understand once you know what each part describes. Learning to read it can help you compare tires and spot a mismatch before buying a replacement.
How to Read a Tire Size
Start with a marking such as P225/65R17 100T. The numbers and letters describe the tire’s type, dimensions, construction, and service ratings. They do not, by themselves, confirm that the tire is suitable for your vehicle. Check the vehicle’s tire placard or owner’s manual before choosing a replacement.
- P: Passenger tire type
- 225: Nominal section width in millimeters
- 65: Sidewall height as a percentage of the section width
- R: Radial construction
- 17: Wheel diameter in inches
- 100T: Load index and speed symbol, together called the service description
What Each Part of the Tire Size Means
Tire type: P, LT, ST, or T
A letter or group of letters before the size can identify the tire’s intended category. For example, P generally indicates a passenger tire, LT a light-truck tire, ST a special trailer tire, and T a temporary-use spare. The exact marking conventions depend on the tire and market, so use the vehicle manufacturer’s approved tire information when selecting a replacement.
Some metric tire sizes have no letter before the width. Do not assume that a missing letter makes a tire interchangeable with a P-metric tire of the same dimensions. Tire type, load capacity, and inflation requirements still matter.
Width: the first number
In 225/65R17, the first number, 225, is the tire’s nominal section width in millimeters. It describes the approximate width between the sidewalls when the tire is mounted on a specified measuring rim. The tire’s actual dimensions can vary somewhat by model and rim width.
Aspect ratio: the number after the slash
The 65 is the aspect ratio. It means the sidewall height is approximately 65% of the tire’s section width. For a 225-mm-wide tire, that works out to about 146 mm per sidewall (225 × 0.65). It is not the tire’s overall height.
A different aspect ratio changes the tire’s dimensions. If you are considering a different size, verify the approved fitment and overall diameter rather than assuming that a tire will work because it clears the wheel arch while parked.
Construction: R or D
R means radial construction, which is used on most modern passenger-car tires. A D can indicate diagonal or bias-ply construction. Do not interpret an “F” before the construction letter as a universal run-flat code. Run-flat markings vary by tire manufacturer; check the tire’s full sidewall labeling and its manufacturer information.
Wheel diameter: the number after the construction letter
The 17 in R17 is the wheel diameter in inches. It identifies the wheel size the tire is designed to fit. It is not the tire’s overall outside diameter.
Load index and speed symbol
The numbers and letter after the tire size are the service description. In 100T, 100 is the load index and T is the speed symbol. They must be considered with the tire type, inflation pressure, and vehicle specifications.
A load index is a code associated with a tire’s load-carrying capacity under specified conditions. For example, index 100 corresponds to 1,764 pounds in the standard load-index table. That number alone is not enough to approve a replacement: tire construction and inflation pressure affect usable capacity. Check the manufacturer’s fitment information and do not install a tire with a lower rating than the vehicle requires.
A speed symbol identifies a tested speed category under specified service conditions. It is not a recommended driving speed, and it does not mean that a vehicle is safe to drive at that speed. Underinflation, overloading, damage, tread condition, road conditions, and the vehicle itself all affect safety.
| Common speed symbol | Speed category |
|---|---|
| Q | 99 mph (160 km/h) |
| S | 112 mph (180 km/h) |
| T | 118 mph (190 km/h) |
| H | 130 mph (210 km/h) |
| V | 149 mph (240 km/h) |
| W | 168 mph (270 km/h) |
| Y | 186 mph (300 km/h) |
This is a selection of common categories, not a tire-buying guide by itself. Follow the vehicle manufacturer’s requirements and any applicable local rules when choosing a replacement.
Two Tire Size Examples
These examples show how the markings fit together. Use them to decode a sidewall, not to decide whether that size is approved for a particular vehicle.
P225/65R17 100T
- P: Passenger tire designation
- 225: Nominal section width of 225 mm
- 65: Sidewall height is about 65% of the section width, or approximately 146 mm
- R: Radial construction
- 17: Fits a 17-inch wheel
- 100T: Load index 100 and speed symbol T
P275/55R20 111S
- P: Passenger tire designation
- 275: Nominal section width of 275 mm
- 55: Sidewall height is about 55% of the section width, or approximately 151 mm
- R: Radial construction
- 20: Fits a 20-inch wheel
- 111S: Load index 111 and speed symbol S
Where to Find the Correct Tire Size for Your Vehicle
Check the tire information placard, usually on the driver’s door edge or door jamb, and consult the owner’s manual. Depending on the vehicle, the placard or manual may list more than one approved size, such as different sizes for front and rear tires.
The tire currently on the vehicle can be a useful reference, but it may have been installed by a previous owner and may not match the manufacturer’s specification. Confirm the size against the placard or manual before ordering. If the wheels or suspension have been modified, ask a qualified tire professional to verify fit, load capacity, and clearance.
The tire placard also lists the vehicle’s recommended cold inflation pressure. Do not use the maximum pressure molded on the tire sidewall as your normal inflation target. For related advice, see our guide to tire sidewall damage and when to replace a tire.
Other Markings on a Tire Sidewall
DOT mark and Tire Identification Number
In the United States, the DOT mark is followed by a Tire Identification Number (TIN). The TIN contains manufacturer and tire information used for identification and recalls. The last four digits of the full code show the week and year of manufacture. For example, 3224 means the tire was made during the 32nd week of 2024. The full date code may appear on only one sidewall, so check the inner side if you cannot see it.
The DOT mark is not a quality ranking or an endorsement of a particular brand. To learn more about tire age and maintenance, check NHTSA’s TireWise tire safety guidance.
UTQG treadwear, traction, and temperature grades
Many passenger tires sold in the United States display Uniform Tire Quality Grading (UTQG) information. These grades can help with comparisons, but they do not cover every aspect of tire performance and are not a guarantee of how a tire will perform in everyday use.
- Treadwear: A comparative grade based on a standardized test. It is not a mileage promise. Actual tread life depends on the tire, vehicle, maintenance, road surface, and driving habits.
- Traction: A grade based on specified wet-braking tests. It does not describe every kind of wet-weather grip, such as cornering or resistance to hydroplaning.
- Temperature: A grade for resistance to heat under specified test conditions. It is not a tire speed rating.
NHTSA explains the limits of these comparisons in its tire safety ratings information. Use UTQG grades as one reference when comparing eligible tires, alongside the vehicle’s required size and service ratings.
Why the Correct Tire Size Matters
A replacement tire needs to fit the wheel and meet the vehicle’s requirements for load and speed. A size that appears to fit can still have the wrong load capacity, interfere with suspension or body components, or change the tire’s overall diameter. Changes in overall diameter can affect speedometer readings and vehicle systems that rely on wheel-speed information.
Do not choose a different size based only on appearance or a calculator’s estimate. If you want to change wheel or tire size, confirm the combination with the vehicle manufacturer’s guidance or a qualified tire professional. If the tires are already showing uneven wear, our guide to tire wear patterns can help explain what to look for.
Frequently Asked Questions
What do the numbers in a tire size mean?
In a size such as 225/65R17, 225 is the nominal section width in millimeters, 65 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. The R indicates radial construction.
How do I know what tire size my car needs?
Check the tire information placard on the vehicle and the owner’s manual. Verify the current tires against that information, especially if the vehicle has had different wheels or tires installed.
Can I use a different tire size if it fits the wheel?
Not necessarily. The replacement must also meet the vehicle’s fitment, load, and service requirements. A different overall diameter or width may affect clearance, handling, speedometer readings, or vehicle systems. Confirm the proposed size with the manufacturer’s specifications or a qualified tire professional.
Is the tire’s maximum pressure the pressure I should use?
No. The maximum pressure printed on a tire sidewall is not the same as the vehicle’s recommended cold inflation pressure. Use the pressure on the vehicle placard or in the owner’s manual.
What does the last four-digit DOT code mean?
The final four digits of the TIN identify the week and year the tire was manufactured. For example, 3224 indicates the 32nd week of 2024. The full date code may be visible on only one sidewall.
