Load index, and why it matters more than width

Nobody quotes their service description. People say "I'm on 285s" and "I run 35s"; nobody has ever said "I'm on 121s."

Which is odd, because of everything moulded into a sidewall, the service description is the only part that states a limit. Diameter and width describe a shape. The two or three characters after the size — 116T, 104H, 121/118R — describe what the casing is certified to carry and at what sustained speed. Everything else on the tire is geometry; this is the specification.

The index is a code, and the code is deliberately not linear

The first part is a load index: a number, usually between about 70 and 130, that resolves through a published series every manufacturer reproduces identically. Index 116 means 1,250 kg, or 2,756 lb, per tire. Index 121 means 1,450 kg, or 3,197 lb.

It is not a linear scale, and that is the first thing to internalise. Going from index 97 to 101 — four steps — adds 210 lb per corner. Going from 116 to 120 — also four steps — adds 331 lb. The series is engineered to give roughly proportional resolution across a very wide range of vehicle weights, so a step near the bottom is a smaller absolute number than a step near the top.

The practical consequence: you cannot eyeball the difference between two indexes. "It's only two numbers lower" means nothing without the table. Type the whole service description into the load and speed index decoder and it comes back as a weight; the full series is on the reference page.

Width buys nothing at all

Now the claim in the title, stated as bluntly as it deserves.

Take 235/70R16, which measures 9.25 in across the casing and stands 28.95 in tall. Set it next to 305/70R16 at 12.01 in and 32.81 in. That is +2.76 in of width and +3.86 in of height: an enormous visual difference, a genuinely different tire, a different wheel requirement, a different price.

If both are marked at load index 116, both carry 2,756 lb. Identically. To the pound.

The wider tire is not stronger. Capacity comes from the casing's construction and its reference inflation pressure, not from its footprint, and the index is the manufacturer's declaration of both. A narrow, tall light-truck tire built to a high load range routinely outrates a much wider passenger tire that looks far more serious. Compare the two sizes directly and every figure that comes back is a dimension — because dimensions are all the size code contains.

This is why the emphasis in most tire conversations is inverted. Width is the visible, discussed, photographed property. Capacity is the one with a number you cannot exceed.

The index is a statement about air

Here is the part that is genuinely under-appreciated: a load index is only true at a stated inflation pressure.

The published rating assumes the casing is holding the reference pressure the standard specifies. A tire below that pressure does not carry its index — it carries less, on a defined curve of pressure against capacity that manufacturers publish as inflation tables. So the index is not a property of the rubber and steel alone. It is a property of the structure plus the air inside it, and one of those two components leaks.

That reframes underinflation completely. A soft tire is not merely a soft tire. It is a downrated tire, still wearing the marking for a rating it is no longer providing. The relationship between pressure and capacity is set out in tire pressure and load tables.

Why one size appears with several different indexes

Because the reference pressure is a design choice, the same nominal dimensions can be built several ways:

• Standard Load is the baseline passenger construction at the standard reference inflation.
• Extra Load, sometimes printed as XL or Reinforced, is the same size certified at a higher reference pressure, which raises its index. Not a stronger-looking tire — a tire allowed to hold more air. See XL and reinforced markings.
• Light-truck load ranges C, D, E and up step the reference pressure higher again, in a different series with a different construction behind it. Load range and ply rating covers the marking.

So two tires with the same three numbers in the size code, on the same shelf, at a similar price, can differ by hundreds of pounds per corner. Nothing in the size distinguishes them. The service description does, and so does the load range letter.

A second marking to know: a pair like 121/118R quotes two indexes rather than one. Use the higher figure for a tire running on its own, and the lower figure once two are mounted side by side on a dual-wheel axle — the inner tire of a pair sheds heat less easily and the two cannot be assumed to divide the load evenly between them.

The comparison is between two labels, and they point in opposite directions

The tire's marking is a ceiling, published by the tire manufacturer, conditional on pressure.

The vehicle's tire and loading information placard states a floor, published by the vehicle manufacturer, for that vehicle at its rated axle loads — and it is quoted for a specific size at a specific cold pressure, which is why changing size can change the pressure the vehicle needs as well as the capacity it requires.

Reading one and not the other is the actual failure mode. The tire's number, on its own, says nothing about whether it suits any particular vehicle; the placard's number, on its own, says nothing about what is currently bolted on. Both are printed, both are legible in about four seconds, and the comparison is arithmetic between two published figures rather than anything this site can compute for you.

Two arithmetic mistakes worth avoiding

Multiplying by four and comparing to gross weight. A set of four at index 116 comes to 11,023 lb between them, and that number looks reassuring next to almost any vehicle's gross rating. But load is not distributed evenly, and the ratings that matter are per-axle. A loaded pickup can sit well under its gross figure while the rear axle is close to its own limit, and the tires on that axle only ever see the axle's share.

Ignoring the passenger-tire divisor. When a P-series passenger tire is fitted to a light truck, van or SUV, the industry convention divides its rated capacity by 1.10 — the tire is being asked to do a job its series was not standardised for. A P-metric tire that appears to have ample capacity on paper may have noticeably less once that divisor is applied, and nothing on the sidewall reminds you of it.

What this actually looks like in practice

Read the whole string, not the first eleven characters. Decode the index rather than comparing index numbers to each other. Note the load range letter, because it tells you which reference pressure the index was quoted at. Then set the number against the vehicle's placard, which is the only document that knows what the vehicle weighs.

None of that is difficult, and none of it involves the width. Which is the point: the dimension everybody talks about is the one with no limit attached, and the marking nobody reads is the one that has a hard number behind it.