Load Index and Speed Rating: Reading the Numbers That Keep You Legal and Loaded

Every tyre on your truck has a job description printed on the sidewall. It’s the short code near the size, something like 295/80R22.5 152/148 M. Most drivers glance past it. But those last few numbers and that single letter decide how much weight the tyre can legally carry and how fast it can safely run while carrying it.

Get them wrong and you’re not just risking early wear. You’re running a tyre outside what it’s built for, and that’s where carcasses overheat and let go. Usually fully loaded, usually at the worst possible time.

What the numbers actually mean

Two things are hiding in that code: the load index and the speed rating.

• Load index is the two- or three-digit number (the 152/148 in the example). It’s a coded figure that maps to a maximum load in kilograms per tyre. Higher number, more load. Where you see two figures separated by a slash, the first is the rating for single fitment (steer, or a super single) and the second is for dual fitment, where tyres share load and each one carries a little less.
• Speed rating is the letter (the M). It’s the maximum speed the tyre is rated to sustain while carrying its rated load. For most commercial truck tyres you’ll see letters like L, M or N, which sit in the range you’d actually run a heavy vehicle at.

The key thing both share: the rating is a maximum, when the tyre is correctly inflated for that load. Drop the pressure and the real-world capacity drops with it. The number on the wall assumes the tyre is set up properly. It isn’t a promise on its own.

Why the load index matters most near max GVM

If you run well under your Gross Vehicle Mass, you’ve usually got headroom and small mistakes get absorbed. The operators who need to care most are the ones running close to max GVM or GCM: tippers, tankers, anyone loading to the limit on every trip.

Here’s the trap. Load is rarely shared evenly. A load shifts forward, a drive axle carries more than the trailer, one tyre on a dual takes the hit when its partner is soft. The rating has to cover the tyre that’s carrying the most, not the average. So when you’re near the limit, the margin you thought you had can quietly disappear on a single axle.

The fix isn’t complicated: your tyres’ combined rated capacity across an axle needs to comfortably cover that axle’s actual load, with the fitment (single or dual) matched to the right column of the load index. That’s an engineering check, not a guess, and it’s worth doing before you commit to a spec, not after a failure.

Speed rating: matching it to your real duty

Speed rating trips people up because heavy vehicles don’t run fast, so it feels irrelevant. It isn’t. The rating is about sustained heat. A tyre rated for a given speed is rated to shed the heat generated at that speed under load. Run consistently at or beyond it, on long, flat, loaded corridor runs where the tyre never gets a chance to cool, and heat builds in the carcass. Heat is what ages and eventually delaminates a tyre from the inside.

For most commercial duties in our patch, a correctly-rated tyre has plenty of margin. The point isn’t to chase the highest letter. It’s to make sure the tyre you fit is rated for the way you actually run, and not, say, a lighter-duty tyre that happened to be the cheapest thing in the right size.

Where operators get caught out

A few patterns we see roll through the workshop:

• Fitting on size alone. The size matches, so it goes on, but the load index is a step below what the axle needs. It’ll run fine light and struggle loaded.
• Mixing ratings across an axle. Two different load indexes on the same dual position means one tyre is doing more than its share. Match them.
• Ignoring the single-vs-dual column. A tyre rated well for duals can be under-spec as a single fitment. Read the right figure for how it’s mounted.
• Assuming a second-hand or reconditioned tyre carries the same rating. It might be perfectly suitable for the duty (quality used tyres often are), but the rating still has to be read and matched, not assumed. That’s part of why a proper inspection matters.

None of this means you need the most expensive tyre on the shelf. A correctly-rated, well-maintained tyre is the engineering-sound choice: the right index and speed rating for your duty, inflated to suit the load, and checked regularly. Over-speccing is money you didn’t need to spend; under-speccing is a tyre working harder than it’s built to.

Read your sidewall, then have the conversation

Walk around your truck and read the codes off the tyres you’re running now. If they don’t clearly cover the weight you carry and the way you run, that’s worth sorting before it costs you a tyre, or a roadside failure fully loaded.

Ruband Tyres has been keeping commercial vehicles on the road since 1998, and as a direct importer we can match the right load index and speed rating to the duty you actually run, not just what fits the rim. For a specification review, a fleet tyre inspection, or an honest second opinion on what your trucks need, call our Melbourne depot at Bayswater North on (03) 9729 8799, or our Hobart depot at Derwent Park on (03) 6272 7500. You can also request a quote and we’ll get back to you.