Tire load range and load index, explained
Two different markings describe how much weight a tire can carry. Load range (the letters C, D, E, F) appears on LT and ST tires and describes the tire's strength class. Load index (a number like 115 or 121) appears on every tire and converts to an exact maximum load in pounds. They overlap, they are not interchangeable, and both only tell the truth at the right inflation pressure.
Load range: the letter ladder
Load range replaced the old ply rating system, and the letters still map to it. A modern load range E tire does not contain ten actual plies; the rating means it has the strength and air capacity equivalent of a ten-ply bias tire from decades ago. What the letter really buys you is a higher maximum inflation pressure, and pressure is what lets a tire carry load.
| Load range | Ply rating equivalent | Typical max pressure (LT tires) |
|---|---|---|
| B | 4 ply | 35 psi |
| C | 6 ply | 50 psi |
| D | 8 ply | 65 psi |
| E | 10 ply | 80 psi |
| F | 12 ply | 95 psi |
| G | 14 ply | 110 psi |
Exact pressures vary by size, so read the sidewall. The pattern to remember: each step up the ladder is roughly 15 psi more ceiling and a stiffer, heavier tire.
Load index: the number that converts to pounds
The load index is the two or three digit number in the service description after the size, for example the 121 in LT265/70R17 121S. It is a code, not a weight, and it converts like this:
| Load index | Max load per tire |
|---|---|
| 110 | 2,337 lb (1,060 kg) |
| 115 | 2,679 lb (1,215 kg) |
| 120 | 3,086 lb (1,400 kg) |
| 121 | 3,197 lb (1,450 kg) |
| 126 | 3,748 lb (1,700 kg) |
The catch: an LT tire only carries its full indexed load at full pressure. Load and inflation tables published by tire makers show the same load range E tire carrying roughly 2,200–2,400 lbs at 40 psi instead of 3,197 lbs at 80 psi. Pressure is not a comfort dial you set once; it is part of the load rating. That is why tow vehicles often run higher rear pressures when loaded and why your door-sticker pressures are a floor for daily use, not a ceiling.
A worked example
Take an LT265/70R17 load range E, load index 121, on a half-ton's rear axle. At 80 psi each tire is rated for 3,197 lbs, so the pair sums to 6,394 lbs, comfortably above a typical rear GAWR around 3,800–4,100 lbs. The tires are not the weak link; the axle and springs are. Now drop those tires to 35 psi for a soft ride and the load tables put each tire nearer 2,000 lbs, a pair around 4,000 lbs, suddenly in the same neighborhood as the axle weight of a loaded, towing truck. Same tire, same truck, very different margin. This is why "I bought E-rated tires" is not the end of the conversation.
The P-metric derating rule
Passenger (P-metric) tires are rated in gentler service. When a P-metric tire is fitted to a truck, SUV, van, or trailer, industry practice divides its rated load by 1.1. A P265/70R17 rated at 2,679 lbs is treated as a 2,435 lb tire on a truck. It is one reason two tires with identical size numbers can have very different real capacity, and why trucks that tow usually belong on LT tires. (Our sidewall guide covers the P, LT, and ST prefixes in full.)
So do you need load range E?
- Probably yes: you tow near your truck's ratings, carry a slide-in camper, or run heavy in the bed most of the time. The pressure headroom is the point.
- Probably not: your half-ton tows a modest trailer a few weekends a year and spends the rest of its life commuting. A C or D often rides noticeably better, weighs less (which helps fuel economy), and costs less, as long as its ratings clear your axle loads with margin.
- Never: go below the load index or range on your door-jamb sticker. Up is a choice; down is a downgrade of a safety rating.