A tire’s sidewall is packed with information, but its size code is easier to understand once each number and letter is decoded. A marking such as P225/65R17 102H describes a tire’s type, width, proportions, construction, wheel fit, load capacity, and speed rating. Knowing how to read tire sizes helps drivers compare replacements, check compatibility, and avoid choosing a tire that does not meet the vehicle’s requirements.
Where to Find Tire Size Information
The tire size is molded into the sidewall, usually in large characters. It may look like P225/65R17, 205/55R16, or LT245/75R16. The complete marking can also include a load index and speed rating, such as 102H.
Before selecting a replacement, drivers should check the vehicle’s tire information placard. In most U.S. passenger vehicles, the placard is on the driver’s doorjamb or near the driver’s door. It lists the manufacturer-recommended tire size and cold inflation pressure. The owner’s manual may provide additional guidance, including approved alternatives for different wheels or driving conditions.
The placard is usually a better starting point than the size of a tire currently on the vehicle. A previous owner may have installed a nonstandard size, or the vehicle may have different front and rear sizes. If those sizes differ, each axle’s requirements should be checked separately.
How to Read a Tire Size Code
Consider the example P225/65R17 102H. The code can be divided into sections:
- P: Tire type or vehicle-use designation.
- 225: Nominal tire width in millimeters.
- 65: Aspect ratio, or sidewall height as a percentage of width.
- R: Radial construction.
- 17: Wheel diameter in inches.
- 102: Load index.
- H: Speed rating.
Not every tire displays all these markings in exactly the same format. Some sizes omit a vehicle-type letter, and certain commercial or specialty tires use different conventions. The numbers and letters are still valuable, but they should be interpreted as a complete specification rather than as isolated measurements.
Tire Type Letters: P, LT, and Other Markings
A letter before the width can indicate a tire’s intended use. P means a P-metric tire, generally designed for passenger vehicles such as cars, minivans, and some light-duty SUVs. A size beginning with LT identifies a light-truck tire, commonly used on pickups and vans when their load or operating requirements call for that construction.
A size with no leading letter may follow a metric or Euro-metric format. That does not automatically make it incompatible with a U.S. vehicle. However, tires with similar dimensions can have different load capacities or pressure requirements, so the vehicle manufacturer’s specification and the tire’s complete service information matter.
Some trailer tires begin with ST, meaning special trailer. These are intended for trailer applications, not as substitutes for passenger-car or light-truck tires. A tire’s intended category should match the vehicle and use recommended by its manufacturer.
Tire Width: What the First Number Means
In 225/65R17, the first number, 225, is the tire’s nominal section width in millimeters. It measures the approximate distance from one sidewall to the other when the tire is mounted on a specified measuring rim and inflated under defined conditions.
For a rough conversion to inches, divide millimeters by 25.4. A 225-millimeter width is about 8.9 inches. That figure is not necessarily the exact tread width: the tread is the road-contacting portion, and its width can differ from the tire’s overall section width.
Actual dimensions can vary slightly by tire model, rim width, inflation, and measurement method. A wider tire is not automatically an upgrade. It must fit the wheel and vehicle without rubbing suspension parts, bodywork, or wheel-well liners, and it must preserve appropriate load capacity and handling characteristics.
Aspect Ratio: The Sidewall Height
The number after the slash is the aspect ratio. In 225/65R17, the 65 means the sidewall height is approximately 65% of the tire’s section width. It is a proportion, not a measurement in millimeters or inches.
In this example, 65% of 225 millimeters is about 146 millimeters, or 5.7 inches. This is an approximate sidewall height, not the tire’s overall height. The full outside diameter also includes the wheel diameter and the sidewall on both sides.
For a tire with the same width, a lower aspect ratio generally means a shorter sidewall, while a higher aspect ratio generally means a taller one. Changing aspect ratio can affect ride comfort, steering response, ground clearance, and the accuracy of the speedometer and odometer. Any alternate size should be approved for the vehicle rather than chosen by appearance alone.
Construction Letter and Wheel Diameter
The R in a modern tire size means radial construction. Radial tires have body cords arranged across the tire, with reinforcing belts beneath the tread. Most tires sold for everyday passenger vehicles in the United States are radial tires.
The number after the construction letter is the wheel’s diameter in inches. A 17 in 225/65R17 means the tire is designed to mount on a 17-inch wheel. This is a required fit dimension: a tire made for a 17-inch wheel cannot be installed on an 18-inch wheel.
Older or specialty tires may use D for diagonal, or bias-ply, construction. The full size may also include a hyphen in older formats. Drivers replacing such tires should consult the vehicle or equipment manufacturer because construction and application requirements can differ.
Load Index and Speed Rating
The numbers and letter after the size are the tire’s service description. In 102H, 102 is the load index and H is the speed rating. The load index is a standardized code corresponding to a maximum load under specified conditions; it is not the number of pounds printed directly on the sidewall.
For example, load index 102 corresponds to a maximum capacity of 850 kilograms, or about 1,874 pounds, per tire under the applicable rating conditions. The load index should be checked against the vehicle’s requirements, and the vehicle’s gross axle and vehicle weight limits still apply. The rating does not mean that a vehicle can safely carry that amount in addition to its own weight.
A speed-rating letter indicates the maximum speed category established under specified test conditions. It is not a recommended driving speed and does not make speeding safe or legal. Tires should meet the vehicle manufacturer’s requirements for speed rating, load capacity, and use. Drivers should not assume that a higher-looking letter alone makes a tire a suitable replacement.
Because load-index and speed-rating codes are standardized, a reliable tire chart or tire professional can confirm the exact values. A replacement should generally have a load capacity and speed rating that meet or exceed the vehicle manufacturer’s specification, unless the manufacturer specifically permits another fitment.
Understanding LT Tire Sizes and Load Range
Light-truck tires may show a format such as LT245/75R16. Here, LT identifies the tire category, 245 is the nominal width in millimeters, 75 is the aspect ratio, R indicates radial construction, and 16 is the wheel diameter in inches.
LT tires may also display a load range, such as C, D, or E. Load range is associated with the tire’s strength and inflation capabilities; it is not a direct measurement of the load that the vehicle should carry. The same load-range letter can correspond to different capacities depending on the tire size and inflation pressure.
For that reason, the sidewall’s maximum pressure should not be used as the everyday inflation target. Drivers should use the cold inflation pressure listed on the vehicle placard unless the vehicle or tire manufacturer gives different application-specific instructions. Increasing pressure without a reason can affect ride and tire wear, while insufficient pressure can cause excessive heat and tire damage.
Other Common Tire Sidewall Markings
Tire sizes appear alongside other markings that help identify the tire and its limits. These are not all part of the size code, but they can be useful when comparing tires or checking service history.
- M+S: A mud-and-snow designation used on some tires. It does not, by itself, certify a tire for severe snow service.
- Three-Peak Mountain Snowflake: A mountain-and-snowflake symbol indicating that the tire meets a recognized performance standard for severe snow conditions. It does not replace careful winter driving or appropriate chains where required.
- Maximum load and pressure: Sidewall information stating limits for the tire under specified conditions. The vehicle placard, not the tire’s maximum pressure, normally provides the correct operating pressure.
- UTQG ratings: Treadwear, traction, and temperature grades shown on many passenger tires. These are comparative ratings under specified test methods, not guaranteed mileage or a complete measure of real-world performance.
- DOT code: A Department of Transportation identification marking that includes information used to identify the tire. Its final four digits indicate the week and year of manufacture.
How to Read a Tire’s DOT Date Code
The date is part of the tire identification number, often called the DOT code. The final four digits indicate the manufacturing week and year. For instance, 3523 means the tire was manufactured during the 35th week of 2023.
The date code may appear on only one sidewall, so it can be necessary to inspect both sides of the tire. A date code identifies when the tire was made; it does not, by itself, establish whether the tire is safe to use. Condition, storage, service history, and the vehicle or tire manufacturer’s guidance also matter. NHTSA advises drivers to monitor tires for wear, damage, and aging, and to follow relevant manufacturer recommendations.
How to Check Whether a Tire Size Fits
A replacement tire should match the vehicle’s approved size and meet its load and speed requirements. A tire that physically fits the wheel may still be unsuitable for the vehicle. Changes in overall diameter can affect clearance, braking and handling systems, and speedometer readings. Differences in width can cause rubbing or alter how the tire behaves.
When comparing an alternative size, drivers should check the following:
- Vehicle placard and manual: Confirm the recommended size, cold pressure, and any approved alternate fitments.
- Wheel compatibility: Verify that the tire is designed for the wheel diameter and an appropriate rim width.
- Load and speed ratings: Make sure the replacement meets the vehicle’s stated requirements.
- Clearance and system compatibility: Check for adequate space around suspension and body components and consider potential effects on vehicle systems.
- All-wheel-drive requirements: Follow the manufacturer’s guidance on matching tire sizes and tread depth, since differences can strain some drivetrains.
A tire retailer can check fitment by vehicle year, make, model, and trim, but the door placard and owner’s manual remain important references. If the vehicle has modified wheels, suspension, or other equipment, a qualified tire professional should confirm the complete setup.
Example: Decoding P225/65R17 102H
Putting the parts together, P225/65R17 102H describes a P-metric passenger tire with a nominal section width of 225 millimeters. Its sidewall height is approximately 65% of that width. It uses radial construction and fits a 17-inch wheel. The 102 load index corresponds to a standardized maximum load of 850 kilograms per tire under the applicable conditions, and H is its speed-rating category.
This description is useful for identifying a tire, but it is not enough by itself to determine whether the tire is right for a particular vehicle. The vehicle’s placard, manual, fitment requirements, and the tire’s other markings should also be checked.
Reliable Sources for Tire Information
For vehicle-specific pressure and size guidance, drivers should begin with the tire information placard and owner’s manual. The National Highway Traffic Safety Administration’s tire safety resources explain tire maintenance and safety basics. Tire manufacturers and qualified retailers can provide product specifications, load tables, and fitment information for a particular tire and vehicle.