Tire Service Description Explained: Reading Load Index and Speed Rating Together

What a tire service description means

A tire service description is the number-and-letter combination molded into the sidewall right after the tire size. In 265/70R17 116T, the size is 265/70R17 and the service description is 116T. The number 116 is the load index and the letter T is the speed rating. Neither part is a measurement you can read directly. Both are codes that point to standardized tables. Load index 116 corresponds to a maximum of 1,250 kg, or about 2,756 lb, per tire. Speed rating T corresponds to 118 mph (190 km/h).

The two halves answer different questions, but they belong together because the speed rating is established with the tire carrying a load at the correct inflation. A tire that meets or exceeds the load index and speed rating specified for your vehicle meets the manufacturer's minimum. A tire with a lower number or a lower letter does not, even if the size is identical. The core rule is simple: match the size, then meet or exceed both parts of the service description.

How load index and speed rating work

The load index is a scale, not a weight, and each step up does not add the same amount of capacity. Here are a few common values from the standard load index chart: 91 is 615 kg (1,356 lb), 94 is 670 kg (1,477 lb), 100 is 800 kg (1,764 lb) and 116 is 1,250 kg (2,756 lb). The sidewall usually also prints the maximum load in pounds and kilograms, together with the inflation pressure at which that load applies. Capacity depends on air pressure, so a tire run below its specified pressure cannot safely carry its full rated load.

Speed ratings mostly follow the alphabet, with H as the historical exception. Common letters on passenger and light truck tires are Q (99 mph), R (106 mph), S (112 mph), T (118 mph), H (130 mph), V (149 mph), W (168 mph) and Y (186 mph). A Y in parentheses means the tire can exceed 186 mph. ZR in the size marks a tire intended for speeds above 149 mph. The rating comes from a laboratory test at a defined load and pressure under controlled conditions. It describes a new tire's capability, not a recommended driving speed, and it never overrides speed limits or road conditions.

What changes the answer

The same load index does not always mean the same practical capacity. P-metric, Euro-metric standard load, extra load (XL or Reinforced) and LT tires reach their rated load at different reference pressures. An XL tire carries more than a standard load tire of the same size only when it is inflated to the higher pressure that the extra capacity requires. LT tires often show two load numbers, such as 121/118S. The first number applies when the tire is mounted singly and the second applies when it runs in a dual-wheel pair. A P-metric tire fitted to a light truck, SUV or van has reduced usable capacity. That is one reason pickups and vans often need LT tires or a higher load index.

If you mix speed ratings on one vehicle, the vehicle is limited to what the lowest-rated tire can handle, and some manufacturers require all four tires to match. Winter tires are a common exception. Some vehicle makers permit a lower speed rating for winter tires, usually with a matching speed restriction, so check the owner's manual instead of assuming. On all-wheel-drive and four-wheel-drive vehicles, drivetrain compatibility adds requirements beyond the service description. Overall diameter and tread depth need to stay close across all tires, because a mismatch can make the drivetrain compensate continuously. A tire with the right code but the wrong size or a very different wear level can still be a poor fit.

What the owner can do

Start with the vehicle placard on the driver's door jamb and with the owner's manual. Between them, they give the original tire size, the cold inflation pressures, and normally the load index and speed rating the vehicle was certified with. Compare that specification with the sidewall of each tire on the vehicle, reading the service description right after the size. Write down both parts for every tire, including the spare if it is a full-size unit.

When you buy replacements, choose the same tire size with a load index and speed rating equal to or higher than the original specification. A higher speed rating is acceptable, although depending on construction it can bring a firmer ride or different wear. Inflate to the placard pressure, not the maximum pressure printed on the sidewall. The exception is an XL or LT substitution that needs a different pressure, which a tire professional can confirm from load and inflation tables. If you tow regularly, haul heavy cargo or often run near the placard's maximum combined occupant and cargo weight, consider a higher load index, never a lower one.

Inspect what the code cannot tell you. Measure tread depth across each tire, look for bulges, cuts or uneven wear, and note how old the tires are. A tire that has been underinflated, overloaded, damaged or repaired may no longer perform to its original rating, and some tire makers treat a repaired tire as having lost its speed rating.

Limitations and next step

A service description sets minimums for a single tire under standardized conditions. It does not account for wear, heat buildup from sustained heavy loads, poor alignment or how a particular vehicle distributes weight between its axles. It also says nothing about rim width compatibility or body clearance, which are separate fitment checks.

Ask a tire professional to verify the fitment against load and inflation tables for your exact vehicle in any of these cases: the placard is missing or unreadable, the vehicle has a lift, different wheels or a non-original tire size, or you are switching between P-metric and LT tires. Trailers, motorhomes and commercial use also call for professional advice, because their load requirements differ. For an ordinary replacement, the rule stays the same: same size, same or higher load index, same or higher speed rating, and inflation set to the vehicle's specification.