How to Read Tire Size, Load Index, Speed Rating and Date Code
A tire code identifies width, aspect ratio, construction and rim diameter, followed by load and speed ratings; the vehicle placard remains the starting point for approved size and cold pressure.
Timeline
- Start with the vehicle: Read the door-jamb Tire and Loading Information label and owner's manual for original size, load and cold-pressure requirements.
- Decode the tire: Confirm size, construction, load index, speed symbol and any vehicle-specific markings.
- Inspect and maintain: Record the DOT identification number, check recalls, pressure, tread, damage and age throughout service.
A code such as 225/45R17 94V is a compact description, not a complete replacement recommendation. In this example, 225 is the nominal section width in millimetres. The number 45 is the aspect ratio: sidewall height is 45 percent of the nominal width. R means radial construction, and 17 is the wheel-rim diameter in inches. Those mixed units are standard industry notation; changing one part can affect clearance, gearing, handling and load capacity. [1][2][3]
The following number, 94 in the example, is a load index rather than pounds or kilograms. A standard table converts the index into the maximum load one tire can carry under specified conditions. The letter V is the speed symbol, which corresponds to a tested maximum speed category under the relevant load. Drivers should match the vehicle maker's requirements and never treat a high speed symbol as permission to exceed road limits or ignore inflation and loading. [2][3][4]
Start replacement shopping with the Tire and Loading Information label on the driver's door edge or pillar and the owner's manual. NHTSA says these give the vehicle manufacturer's correct tire size and recommended cold inflation pressure. Vehicles may specify different front, rear or spare sizes and pressures. If wheels, suspension or intended use have changed, consult the vehicle and tire manufacturers or a qualified professional rather than assuming that any tire fitting the rim is suitable. [1][5]
The maximum pressure moulded on a tire's sidewall is not the routine pressure target for the vehicle. Use the manufacturer's placard value measured when the tires are cold, commonly before driving or after the vehicle has been parked for several hours. Check all tires, including a spare, with a gauge. A tire-pressure monitoring warning typically indicates significant underinflation; it does not replace regular checks or prove that every tire is at its recommended value. [1][5]
The DOT Tire Identification Number links a tire to its manufacturing plant and specification and helps identify recalls. Its final four digits give the production week and year: NHTSA's example 0308 means the third week of 2008. The complete code may appear on only one side, so inspect both sidewalls. Age alone is not the sole replacement test, but NHTSA notes that some vehicle and tire manufacturers recommend replacement after six to ten years regardless of tread. [1][6]
Other sidewall markings serve different purposes. U.S. passenger tires may display Uniform Tire Quality Grading for relative treadwear, wet-traction performance and temperature resistance. These grades support comparisons under the programme's test conditions; they are not a warranty of exact mileage or all-weather grip. Markings such as M+S, severe-snow symbols, run-flat identifiers, extra-load designations and original-equipment codes should be checked against the intended vehicle and climate rather than inferred from size alone. [1][2][3]
Before buying, compare the full service description, not only width and rim diameter, then check for recalls using NHTSA's tire or equipment search. During use, inspect monthly for pressure, tread depth, cuts, bulges, cracking and irregular wear, and seek professional help for vibration or repeated pressure loss. A correct sidewall code cannot make a damaged, overloaded or underinflated tire safe. The vehicle placard, tire markings and physical condition answer separate parts of the safety decision. [1][5][6]
Sources
- Tire Safety Ratings and Awareness
- Continental Tire Markings Guide
- Michelin Tire Markings Explained
- Continental Tire Load Index Guide
- NHTSA Tire and Loading Information Guidance
- NHTSA Recalls Search