A tire sidewall is more than a string of letters and numbers: it records the tire’s size, construction, capacity, and manufacturing date. Learn to decode those markings so you can compare a replacement with your car’s specifications and spot details that deserve a closer look.
Start with the full tire code
Find the largest group of characters molded into the sidewall. A common passenger-car example is P205/55R16 91V. The letters and numbers describe several different things; read the whole code before comparing tires.
| Marking | Example | What it tells you |
|---|---|---|
| Tire type | P | Intended service category in the U.S. |
| Width | 205 | Nominal section width in millimeters |
| Aspect ratio | 55 | Sidewall height as a percentage of width |
| Construction | R | Radial construction |
| Wheel diameter | 16 | Rim diameter in inches |
| Load index | 91 | Maximum load rating under specified conditions |
| Speed symbol | V | Speed capability category under specified conditions |
The example is illustrative, not a recommendation for any particular car. Sidewall codes and available markings vary by tire type, market, and manufacturer. Your owner’s manual, tire placard, and tire maker’s documentation take priority when choosing a replacement.
Type, width, and aspect ratio
A leading P identifies a P-metric tire intended for passenger-car service. Some tires omit it and use a metric designation; LT indicates a light-truck tire, while ST is used for special-trailer tires. These categories are not interchangeable just because the dimensions look similar. Confirm the vehicle and tire makers’ specifications before substituting one type for another.
In 205/55R16, 205 is the nominal width from sidewall to sidewall in millimeters. The number after the slash is the aspect ratio: the sidewall height is nominally 55 percent of the section width. A lower ratio usually means a shorter sidewall relative to width; it does not alone tell you whether a tire will fit, ride well, or be suitable for your car.
Construction and wheel diameter
R means radial construction, the usual design on modern passenger cars. A D or B in some size codes can identify other construction types, such as diagonal or bias-belted construction. Classic cars may have original equipment or restorations that call for a specific construction. Do not assume a modern radial is an acceptable substitute solely because it can be mounted on the same-diameter wheel; check vehicle and tire manufacturer guidance, including any requirements for mixing constructions.
The final number in the size portion is the wheel diameter in inches. A 16-inch tire is made for a 16-inch-diameter rim. It is not the tire’s outside diameter, and it will not fit a 15-inch wheel. Verify the wheel diameter stamped or specified for the wheel, rather than estimating it from the car’s appearance.
Understand load index and speed symbol
After the size, a marking such as 91V typically combines a load index and speed symbol. The load index is a code, not a number of pounds. Look it up in the tire manufacturer’s load-index table to find its rated capacity, and make sure the replacement meets the vehicle maker’s required load capacity. The rating applies under defined conditions, including specified inflation pressure; it does not mean a tire can safely carry that load at any pressure.
The speed symbol indicates a standardized speed capability category under specified test conditions. It is not a recommended driving speed, a guarantee of performance in every situation, or permission to exceed posted limits. Heat, tire condition, pressure, load, road surface, and vehicle setup affect real-world safety. Match the vehicle maker’s specification and consult a tire professional if a proposed replacement changes the rating.
Extra markings for trucks and trailers
Light-truck sizes may include LT and additional load-range or service-description markings. Trailer tires may carry ST and a load range. Those details matter: a tire sized for a passenger car is not automatically suitable for a loaded pickup, and a trailer tire is designed for trailer service rather than steering or drive use. Use the vehicle or trailer placard, manual, and tire maker’s fitment information to confirm the correct type and capacity.
Find the right inflation information
The sidewall often shows a maximum pressure, sometimes alongside a maximum load. That is a tire limit under stated conditions, not the everyday pressure target for your car. For routine cold inflation pressure, check the vehicle’s tire information placard—often on the driver’s door jamb—or the owner’s manual. A placard may list different pressures for front and rear tires or for specified loads.
Check pressure when tires are cold, following the vehicle and tire maker’s instructions. A tire that has been driven on can read higher as it warms; do not bleed air from a warm tire simply to match the cold-pressure figure. Use a reliable gauge, and replace valve caps after checking. If pressure keeps falling, inspect for damage and arrange a professional assessment rather than repeatedly topping it up and ignoring the cause.
The sidewall’s maximum pressure should not be used as a shortcut when a placard is missing or unreadable. For an older car, consult its factory documentation, a reputable tire manufacturer’s fitment guidance, or a qualified tire shop familiar with the vehicle and its wheels. Restored cars, modified suspensions, changed wheel sizes, and nonstandard tire types may not match the original placard setup.
Decode the date and identify the tire
Look for a molded DOT identification string. The full code can include information identifying the plant and tire specification; the final four digits of the date code indicate the week and year of manufacture. For example, 2324 means the 23rd week of 2024. Some tires show the complete date code on only one side, so you may need to inspect the inward-facing sidewall or ask a shop to read it.
A date code is not a simple pass/fail verdict on tire safety. Aging depends on storage, use, heat, sunlight, maintenance, and the tire maker’s guidance. A tire can be damaged before a particular age, while a date alone cannot certify that it remains safe. Follow the vehicle and tire manufacturers’ inspection and replacement recommendations, and have an older tire assessed by a qualified tire professional. This is especially prudent for a classic car that spends long periods parked or is driven infrequently.
Other sidewall text may include the brand, model, M+S marking, or a mountain-and-snowflake symbol. These identify product or service characteristics, but do not replace a check of the tire’s condition or suitability for local weather and the vehicle. A marking is useful evidence; it is not a complete maintenance history.
Check fit before buying or mounting
Use this practical sequence when comparing a replacement:
- Read the vehicle specification. Check the placard and manual for the approved size, load index, speed rating, and pressure. Note any alternate sizes listed by the vehicle maker.
- Compare the complete code. Confirm tire type, width, aspect ratio, construction, and rim diameter, then compare load and speed ratings. A partial match can conceal a meaningful difference.
- Confirm wheel compatibility. The tire must be approved for the wheel’s diameter and width. A correct diameter alone does not establish that the wheel and tire are compatible.
- Check the whole setup. Different sizes can change overall diameter, clearance, speedometer indication, handling, gearing, and the operation of vehicle systems. Do not rely on visual similarity; use manufacturer-approved fitment data.
- Ask about the vehicle’s use. A classic driven gently on the street, a heavy pickup, and a car used for track events have different needs. Share the vehicle, wheel, load, and intended use with a tire professional.
For a classic car, the original tire size may be written in an older format or shown in period documentation rather than as a modern metric code. A modern replacement’s sidewall may not resemble the original marking. Use a conversion or fitment recommendation from a tire manufacturer or specialist, and verify the rim type and width. A conversion is a starting point, not proof that a tire suits the car’s load, clearance, speed capability, or intended use.
Do not mix tire sizes, constructions, or markedly different capabilities across an axle or vehicle unless the vehicle maker specifically permits the arrangement. Some cars have different approved front and rear sizes; follow those specifications. If a car has modified wheels, suspension, or brakes, fitment may require an expert check beyond decoding the sidewall.
Inspect the sidewall during routine care
When checking pressure, look over the visible sidewalls as well as the tread. Watch for cuts, cracks, bulges, exposed cords, impact marks, or a tire that looks distorted. A bulge can indicate internal damage; avoid driving on a tire with a bulge and arrange professional help. Do not attempt a sidewall repair with a plug or sealant unless the tire maker or a qualified professional specifically confirms an appropriate repair method. Sidewall damage often cannot be safely repaired.
Check the inner sidewall too, if it is safely visible. Do not crawl under a vehicle supported only by a jack, put your body in the path of a vehicle, or reach near moving parts. If you cannot see the inner side, have a shop inspect it during service. A slow leak, repeated pressure loss, vibration, or a pull that persists after basic checks calls for inspection; the sidewall markings alone cannot diagnose the cause.
Keep tires clean with methods approved by the tire manufacturer. Avoid harsh solvents and dressings unless the manufacturer says they are suitable. If a car is stored, follow its manual and tire-maker advice on pressure, movement, and support; a cover or tire product cannot reverse age or structural damage. For equipment-specific instructions, such as a compressor or pressure gauge, follow that equipment’s manufacturer manual.
Troubleshooting common sidewall questions
The code does not match the tire I expected
First check that you are reading the full size rather than a nearby load, pressure, or product marking. Then compare it with the placard and manual. The car may have non-original wheels or tires, but that does not make the current setup correct. Ask a tire shop to verify the wheel and vehicle fitment before ordering.
I can see only part of the DOT code
The date portion may be on the other side. If reaching it means unsafe access, do not move or lift the vehicle yourself just to read the code. A tire shop can inspect it. Record the visible brand, model, and size for reference, but avoid treating an incomplete code as a confirmed manufacturing date.
The pressure on the tire is higher than the door placard
That is common because the sidewall figure is generally a maximum, while the placard gives the vehicle’s recommended cold pressure for the specified setup. Use the placard or manual. If your tire size or load range differs from the factory setup, get a vehicle-specific recommendation rather than guessing from the sidewall maximum.
A tire is old but looks fine
Appearance cannot reveal every internal problem, and a date code cannot establish condition by itself. Check the makers’ age and inspection guidance and have a professional examine tires with uncertain history, long storage, cracking, or other concerns. Do not rely on tread depth alone to judge an old tire.
A quick garage reference
Before a road trip or a tire purchase, write down the complete size, load index, speed symbol, and date code from each tire, then compare them with the car’s documentation. Note any differences between axles and any visible damage. Keep the placard pressure and your gauge with your maintenance notes; the sidewall tells you what tire you have, while the vehicle documentation tells you which setup and cold pressure the car is designed to use.