Load Index and Speed Rating Decoder
The two numbers and one letter after the size constrain more than anything else printed on the sidewall, and almost nobody can read them.
On a sidewall reading LT285/70R17 121/118S, the service description is 121/118S. The pair of numbers is single-wheel then dual-wheel load index.
Load index 116
2,756 lb
1,250 kg per tire at the tire’s rated inflation pressure. Four tires at this index total 11,023 lb.
Speed symbol T
118 mph
190 km/h. Commonly seen on family sedans and vans.
Load indexes around this one
| Index | Per tire | Kilograms |
|---|---|---|
| 110 | 2,337 lb | 1,060 kg |
| 111 | 2,403 lb | 1,090 kg |
| 112 | 2,469 lb | 1,120 kg |
| 113 | 2,535 lb | 1,150 kg |
| 114 | 2,601 lb | 1,180 kg |
| 115 | 2,679 lb | 1,215 kg |
| 116 | 2,756 lb | 1,250 kg |
| 117 | 2,833 lb | 1,285 kg |
| 118 | 2,910 lb | 1,320 kg |
| 119 | 2,998 lb | 1,360 kg |
| 120 | 3,086 lb | 1,400 kg |
| 121 | 3,197 lb | 1,450 kg |
| 122 | 3,307 lb | 1,500 kg |
Every speed symbol
| Symbol | km/h | mph |
|---|---|---|
| L | 120 | 75 |
| M | 130 | 81 |
| N | 140 | 87 |
| P | 150 | 93 |
| Q | 160 | 99 |
| R | 170 | 106 |
| S | 180 | 112 |
| T | 190 | 118 |
| U | 200 | 124 |
| H | 210 | 130 |
| V | 240 | 149 |
| W | 270 | 168 |
| Y | 300 | 186 |
| (Y) | 300 | 186 |
What you are looking at
A full sidewall marking reads something like LT285/70R17 121/118S. The size is the first part.
The rest — 121/118S — is the service description, and it is the part that decides what the tire
can actually carry and how fast it is rated to run.
The number is the load index, and it maps to a maximum load through a standardised published table. It is not linear and it is not a percentage: index 100 is 1,764 lb and index 110 is 2,337 lb, so ten steps is a third more capacity.
Where two numbers appear separated by a slash, the first is the single-wheel rating and the second is the dual-wheel rating, which is lower because tires mounted in pairs share load unevenly and run hotter.
The letter is the speed symbol, which maps to a maximum sustained speed.
The oddity in the speed table
Speed symbols are alphabetical, except that H is not. The order runs Q R S T U H V W Y, with
H at 210 km/h sitting between U at 200 and V at 240. That is not a mistake in the table on
this page. H was standardised before the rest of the series and kept its position when the
others were added around it.
What the ratings mean, precisely
A load index is a maximum at the tire's own rated inflation pressure, cold. Under-inflate it and the real capacity is lower — pressure carries the load, not rubber.
A speed symbol is a rating for sustained running under specified test conditions on a correctly inflated, undamaged tire. It is not a promise about a tire that is worn, under-inflated, or carrying its maximum load on a hot day.
Reading it against a vehicle
Your vehicle's tire and loading information placard, on the driver's door jamb, states the size and the minimum load rating the vehicle was delivered with. A tire's sidewall states what that tire provides. Comparing the two is arithmetic and this page can help with it.
Deciding whether a particular tire suits your vehicle is not arithmetic. It depends on the vehicle's own requirements, how it is loaded, and local rules, and this site does not make that call.
Extra Load, and why the same size can carry more
An XL or Reinforced marking means the same casing size is rated at a higher reference
inflation — 41 psi rather than 35 — which raises its load index without changing its dimensions.
Two tires with an identical size code can therefore have different load indexes, and the one you
need is decided by the placard, not by the shape.