Why load rating matters more than tyre size
It's easy to focus on tyre size when choosing rubber for a RAM, Silverado or F-Series — especially if you're also fitting a lift kit or want a particular look. But the number that actually determines whether a tyre is safe and legal for your truck is the load index (load rating), which tells you the maximum weight the tyre can carry at a specified pressure. Get this wrong and you can end up with a truck that looks right but is under-tyred for its actual GVM or towing capacity.
This matters more on American trucks than on most passenger vehicles because of the weights involved — a loaded RAM 2500 or Silverado 2500HD towing a caravan can be operating close to its GVM and GCM regularly, not just occasionally.
Understanding load index and load range
Every tyre has a load index number moulded into the sidewall alongside the size, representing the maximum load the tyre can carry per tyre at maximum inflation pressure. Light truck (LT) tyres also carry a load range designation (commonly C, D or E) which relates to ply rating and the pressure needed to reach maximum load capacity — a higher load range generally means a higher maximum pressure and higher load capacity, but also a firmer ride unloaded.
| Load range | Typical use | Ride characteristic unloaded |
|---|---|---|
| Standard load (P-metric) | Light-duty RAM 1500/Silverado 1500 without heavy towing | Softest ride |
| LT Load Range C | Half-ton trucks with moderate towing or load | Moderate |
| LT Load Range D | Half-ton trucks towing regularly, light HD use | Firmer |
| LT Load Range E | HD trucks (2500/3500 series) and heavy regular towing | Firmest unloaded, most capacity |
As a general rule, HD-series trucks (RAM 2500/3500, Silverado/Sierra 2500HD/3500HD, F-250/F-350) are factory-fitted with Load Range E tyres because they're regularly required to carry or tow near their rated capacity. Downgrading to a lower load range tyre — even in the correct physical size — can leave the truck under-tyred for its GVM.
The problem with oversized tyres on lifted trucks
Fitting larger tyres as part of a lift kit is common, but it's worth checking the load rating of the new tyre against the original fitment, not just the size. Some larger tyres — particularly all-terrain and mud-terrain designs marketed on looks rather than load capacity — carry a lower load index than the factory tyre they replace. That's a genuine safety issue on a truck that tows or carries load regularly, and it can also affect insurance and compliance if the vehicle is later found to be under-tyred for its GVM.
Larger, heavier tyres also change the truck's rolling resistance, gearing feel and fuel consumption — see our guide to American truck fuel economy for more detail on that side of the equation.
Speed rating and towing
Light truck tyres typically carry a lower speed rating than passenger tyres, which is generally fine for towing speeds but worth checking if you regularly drive at higher highway speeds unladen. The tyre placard on the truck (usually on the driver's door jamb) specifies the correct size, load range and recommended pressures for both laden and unladen use — this is the reference point, not the tyre shop's default recommendation.
Correct tyre pressure for load
- Use the pressures on the door jamb placard as your starting point, not the maximum pressure moulded on the tyre sidewall
- Increase pressure toward the laden setting when towing or carrying a heavy load, as specified by the manufacturer
- Check pressure when tyres are cold, not immediately after driving
- Recheck pressure after any change in load, tyre size or load range
- Have pressure and tyre condition checked as part of every service, particularly before a long tow
An under-inflated or under-rated tyre on a loaded truck is a genuine safety risk, not just a fuel economy or wear issue — it affects braking distance, heat build-up and blowout risk, especially towing at highway speed in Australian heat.
Choosing tyres for towing and work use
If your truck is used mainly for towing a caravan or trailer, prioritise load rating and a highway or all-terrain tread pattern with a good reputation for heat management over an aggressive off-road tread that isn't needed for your use. For trucks doing genuine off-road or work-site duty, a properly load-rated all-terrain or mud-terrain tyre is appropriate, but check load index against your GVM regardless of tread pattern.
Tyre and load advice on the Sunshine Coast
American Truck Centre at 46 Regent St, Caloundra can advise on correct tyre load rating and pressure for RAM, Chevrolet Silverado, GMC and Ford F-Series trucks, including trucks that have been lifted or fitted with non-standard tyres. Call 07 5491 2199 if you're unsure whether your current tyres suit your truck's GVM and towing use.
Frequently asked questions
- What is tyre load rating and why does it matter?
- Load rating (load index) is the maximum weight a tyre can carry at its specified pressure. It matters because a tyre with too low a load rating for your truck's GVM can be unsafe when carrying load or towing, regardless of whether it's the correct physical size.
- What load range tyre does my truck need?
- HD-series trucks like the RAM 2500/3500, Silverado 2500HD/3500HD and F-250/F-350 are typically fitted with Load Range E tyres from the factory. Half-ton trucks like the RAM 1500 and Silverado 1500 usually run Load Range C or D depending on the model and use.
- Can I fit smaller or lower-rated tyres to my truck?
- Not without checking the load rating against your truck's GVM and typical use. Fitting a tyre with a lower load index than the factory specification can leave the vehicle under-tyred, which is a safety and potential compliance issue.
- Where do I find the correct tyre size and pressure for my truck?
- The tyre placard, usually located on the driver's door jamb, lists the manufacturer-specified size, load range and recommended pressures for both laden and unladen driving. This takes precedence over the maximum pressure printed on the tyre sidewall.
- Do oversized tyres on a lifted truck affect load capacity?
- They can. Some larger tyres, particularly aggressive all-terrain or mud-terrain designs, carry a lower load index than the factory tyre they replace, which can reduce the truck's effective load capacity even though the tyre is physically bigger.
- How does tyre choice affect towing?
- Correctly load-rated tyres with good heat management are important for towing, since towing loads the tyres for extended periods at highway speed. Load range, pressure and tread design all play a role in safe, stable towing.
- How often should tyre pressure be checked?
- Check monthly when cold, and always before a long tow or after any change in load. Tyre pressure and condition should also be checked as part of every scheduled service.
- Can American Truck Centre advise on tyre load rating?
- Yes, we can advise on correct tyre load rating and pressure for RAM, Chevrolet Silverado, GMC and Ford F-Series trucks at our Caloundra workshop. Call 07 5491 2199 for advice.
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