What does load index 121 mean?
Answer
3,197 lb per tire
Load index 121 means one tire is rated to carry 3,197 lb — 1,450 kg — at the inflation pressure printed on its own sidewall. Four of them total 12,787 lb.
The working
- Per tire
- 3,197 lb / 1,450 kg
- Four tires
- 12,787 lb
- One index step up
- 3,307 lb
- One index step down
- 3,086 lb
The number you will usually see next to another number
Light-truck tires in this class are commonly marked with two indexes separated by a slash — 121/118 being a typical pairing. The higher figure applies when the tire is fitted singly, one per hub. The lower one applies when it is fitted in pairs on one end of an axle, because two tires side by side heat each other, deflect against each other, and cannot be assumed to share load evenly. Index 118 is 1,320 kg, so the derating in that example is 130 kg per tire.
A single-index marking on a tire of this capacity is not an error; it means the tire is not rated for dual fitment.
The steps here are 50 kg
120 is 1,400 kg, 121 is 1,450, 122 is 1,500. The series has widened from the 30 and 35 kg increments used lower down, so a one-point difference near the top of the chart carries roughly twice the weight of a one-point difference in the nineties. Index differences are not comparable across the table.
What 1,450 kg per tire implies
3,197 lb each, and 12,787 lb across four — figures that belong to heavy-duty pickups, vans and trailers rather than to anything passenger-derived. Reaching them requires a stiff casing and a high inflation pressure, and the rating holds only at the pressure printed on the sidewall. Running such a tire at a pressure chosen for ride comfort puts it outside the conditions the index was awarded under.
Not a property of the size
As everywhere on the chart, the index belongs to the individual tire rather than to the size code it is printed beside. The same size can be produced in more than one load range, and each will carry its own index and its own reference pressure. Reading a capacity off a size code is guesswork; reading it off the sidewall is not.
Reading the rest of the marking
The load index is followed immediately by the speed symbol, which at this capacity is usually a low one. Decode a complete description, or see load range and ply rating for the separate marking that describes the casing's pressure class.