Tire Service Description: Load Index and Speed Rating Combined

What is a service description?

A service description is the combined load index and speed symbol that appears on a tire's sidewall after the size designation. For example, in a size like 225/60R17 99H, the "99H" is the service description: 99 is the load index and H is the speed symbol.

Federal standards define load rating as the maximum load a tire is rated to carry for a given inflation pressure, and they set labeling requirements so you can identify what a tire can do. The service description is a shorthand way to state that information.

You will find the service description on nearly every passenger and light truck tire, and it is one of the key checks when you are matching a replacement tire to your vehicle. For the full breakdown of the size portion, see our tire size explained article.

• Load index: a number that represents a specific load rating in pounds or kilograms.
• Speed symbol: a letter from A to Y that corresponds to a rated speed capability.
• The service description is always shown together, with the load index first and the speed symbol second.

Load index: what the number means

The load index is a numerical code that tells you the maximum load a tire can carry when inflated to its maximum permissible inflation pressure. Federal regulations define load rating as the maximum load for a given inflation pressure, and the maximum load rating is the rating at the maximum pressure. The load index simply maps to that rating.

A higher load index means a higher load capacity. The exact load in pounds or kilograms for each index number is published in industry standards, but you do not need to memorize those tables. What matters is that you choose a tire with a load index at least as high as the one your vehicle manufacturer specifies.

Your vehicle's placard, typically located on the driver's side door jamb, lists the original tire size and service description. Federal standard 571.110 requires that the vehicle maximum load on the tire must not be greater than the maximum load rating marked on the sidewall. So when you replace tires, the new load index must be equal to or greater than the original.

For a deeper look at how load ratings work, see our article on tire load index ratings and what is tire load range.

Speed symbol: what the letter tells you

The speed symbol is a letter that indicates the maximum speed capability for which the tire is rated. The symbol ranges from A (low speed) to Y (over 186 mph), with some letters reserved for specific categories. While the exact speeds are standardized, the key point is that the vehicle's original tire specification includes a speed symbol that matches the car's designed performance.

Federal motor vehicle safety standards do not directly specify speed symbols, but they require tires to meet performance tests such as the high speed performance test in 571.109 and 571.119. That test is conducted at speeds related to the tire's rating, so the speed symbol is a way to confirm that the tire was built for a certain speed range.

When you choose a replacement, you do not need a higher speed symbol than original, but you should not go lower if you ever drive at the speeds the symbol covers. For most daily driving, an H or V rating is common, but your vehicle's placard is the authority.

We explain this in more detail in our tire speed rating guide.

How to use the service description when buying tires

When you shop for new tires, match the service description on the replacement to the one listed on your vehicle's placard or in your owner's manual. This is not just about size: the load index and speed symbol ensure the tire can handle the weight of your vehicle at its intended speeds.

Federal standards 571.110 and 571.120 require that the sum of the maximum load ratings of the tires on an axle must be at least the gross axle weight rating (GAWR) of that axle. If you install a tire with a lower load index, you could overload the tire, which may lead to failure. The service description is the quickest way to verify you are staying within the required load capacity.

Also check the inflation pressure. The load rating of a tire is tied to inflation pressure, so always inflate to the vehicle manufacturer's recommended cold pressure, not the maximum pressure printed on the sidewall. Our tire pressure basics article has more on this.

For vehicles that carry heavy loads, such as trucks and SUVs, you may need an extra load (XL) tire or a light truck (LT) tire with a higher load range. The service description alone may not capture the full load range for LT tires, but it is still a useful comparison. See our piece on extra load tires XL for the difference.

Common misconceptions about service descriptions

A higher speed symbol does not mean the tire has better traction or lasts longer. It only means the tire is rated to sustain a higher speed. The load index and speed symbol are independent: you could have a high load index with a modest speed symbol, or vice versa.

Some drivers assume that all tires of the same size have the same service description. That is not true. Two tires with identical dimensions, such as 225/60R17, may have different load indices because of their construction and intended use. Always compare the full sidewall code.

Another mistake is ignoring the service description when installing tires on a vehicle that has been modified, such as lifted or used for towing. The load on each tire changes, and you may need a higher load index to keep within the GAWR. Consult the vehicle's certification label and, if needed, a tire professional.

You can learn more about matching tires to vehicle loads in our tire vehicle load inflation article.

Where to find the service description on the tire

The service description is printed on the tire sidewall, usually right after the size designation. For example, P235/75R15 105S would have "105S" as the service description. It is part of the tire's required labeling under federal standards, so it must appear on every tire sold for use on public roads.

The load index and speed symbol are always shown together, but they are sometimes separated by a space or a letter for the tire type. For instance, a light truck tire might show LT235/85R16 120/116Q, which has a dual load index for single and dual wheel use. In that case, the first number is for single rear wheel applications and the second for dual.

If you cannot find the service description on the tire, check the vehicle placard or owner's manual. That will list the original equipment size and service description. When you replace tires, keep that code in mind, and if you are unsure, our tire sidewall markings guide explains the whole sidewall.

For a visual example of how service descriptions differ across models, look at our comparisons of 16-inch tires and 17-inch tires. You will see that the load index and speed symbol change depending on the tire's intended application.

What to pick for your use

Questions

What happens if I use a tire with a lower load index than the original?

The tire may be overloaded, which can lead to excessive heat buildup, reduced handling, and potentially a blowout. Federal standards require that the vehicle maximum load on a tire does not exceed the tire's maximum load rating, so you should always use a load index equal to or higher than the original.

Can I use a tire with a higher speed symbol than the original?

Yes, a higher speed symbol means the tire is rated for a higher speed, so it is safe to use at the speeds your vehicle is capable of. It does not harm anything, but it may come with a firmer ride or a different tread design. You should never go lower than the original speed symbol.

Is the service description the same on all tires of a given size?

No. The service description is determined by the tire's construction and intended use. Two tires with the same size designation can have different load indices and speed symbols, so you must compare the full code when choosing replacements.

Does the service description affect the tire's overall diameter?

No, the service description does not affect tire dimensions. It only specifies load capacity and speed capability. The size designation (e.g., 225/60R17) determines the dimensions. However, changing to a different service description may require a different inflation pressure to achieve the required load rating.

Where can I find the recommended service description for my vehicle?

Your vehicle's tire information placard, usually on the driver's side door jamb, lists the original tire size and service description. You can also find it in the owner's manual or on the vehicle's certification label. Always use that code as the baseline for replacement tires.