There you are, shopping for new tires, and you start to find that you don’t really know how the specifications offered relate to what you need. There’s so many different numbers and letters trying to tell you something, but what exactly is it? Out of context tire load ratings mean nothing, so let’s put them into context for those shopping for tires for their 4×4.
Load Range, Speed Rating, Load Index, and Carrying Capacity Explained
There’s a lot of terminology out there used by tire manufacturers that isn’t clearly defined by retailers. Here are some common terms you’ll find when tire shopping:
Load Range: Load range indicates the tire’s weight capacity (as a range, not as a specific number) at a predefined pressure. You can use Load Range to compare similar tires (in terms of size, tread type, etc.), but you wouldn’t want to look at load range figures on different tire sizes or types.
Speed Rating: The speed rating tells you the tire’s safe top speed. For example, if a tire is Z rated, the maximum speed is 149 mph. Most light truck and SUV tires are “N” or “P” ratings, which correspond to 87mph and 94mph, respectively. Anything between P and Z on a truck or SUV is probably overkill (unless you’re racing your 4×4, that is).
The best way to think of speed rating is “heat resistance” – the higher the rating, the hotter the tire can get without failing.
Load Index: The load index refers to the actual load carrying capacity of a tire at a specified pressure. While “load range” is just a proxy for tire strength, load index is the “real McCoy” when it comes to designating a tire’s load carry capacity. A load index of 110 (for example) indicates that a tire can hold 2,340lbs.
|Load Index||Weight Rating||Load Index||Weight Rating|
If you’ve got a truck with a GVWR of 10,000lbs (for example), you need 4 tires that can hold at least 2,500lbs each…which means you need four tires with a load range of 112 or higher (at least if you intend to fully load your truck).
Load Carrying Capacity: Load carrying capacity is the maximum weight the tires can handle and is determined by the load index, tire pressure, and tire temperature. Load Carrying Capacity is not a ‘spec’ that is indicated on any tire – it’s something you have to estimate (don’t worry, estimation is easy).
So how do load index, load range, and speed rating all interact? Essentially, these are all metrics you can use to compare and contrast tires. If you’re looking at two similar tires (in terms of type and size), different load index ratings or load ranges indicate differences in tire strength. Different speed ratings indicate different heat ratings for the tires, etc.
Warning! Load Carrying Capacity Can Change
The ultimate goal of tire load rating is to match your tire’s load carrying capacity to your 4×4, especially during hauling and towing. There are three important things to remember about tire load carrying capacity:
- The carrying capacity is determined at operating temperature. Load Index is not.
- Higher speeds mean more heat and – as a result – less load carrying capacity
- Weight also leads to heat, and also decreases the tire’s load carrying capacity…the more weight you add, the lower the tire’s maximum rating.
Unfortunately, it’s not really possible to determine the load carrying capacity of a tire in real-world use. Instead, we “guess” by purchasing tires with a load index that is 20% higher than required. To use our previous example, let’s say you have a truck with a 10,000lbs GVWR (gross vehicle weight rating). The tires should be able to carry at least 2,500lbs each, plus 20%. That calculates to a 3,000lbs weight rating, or a load index of 119.
By over-estimating your tire’s load rating, you ensure that your truck or SUV won’t experience a dramatic failure at highway speeds when loaded up.
Sometimes, Passenger Tires Can Haul More
You’d think that internal air volume is proportional to the load carrying capacity, but that’s not really true. When you compare tires of the same size to each other, it’s not uncommon to have different pressures and load capacity ratings (higher pressure = lower load carrying capacity). Tire construction does, however, impact heat generation and dissipation. This does change the load carrying capacity from one tire to the next.
If, for example, you have two 275/65 tires, but one starts with a “P” and the other an “LT”, the P tire is going to have a 10% higher load carrying capacity at its max pressure. This is because P-metric and LT tires are two very different tires.
LT construction has more rubber, resulting in heavier plies, deeper tread patterns, and stronger bead bundles. These make more heat, and that goes back to the prior discussed impact of heat on load carrying capacity. Therefore, it’s a mistake to assume LT tires can haul more weight than P tires. Sometimes, a strong P tire is going to be great for hauling and towing (depending on the vehicle).
Of course, a P tire isn’t going anywhere off-road, so who cares, right?
Mudding and Hauling Aren’t Always Compatible
4×4 owners often notice that mud tires aren’t available in E or higher — this is due to the tire construction. The flotation sized tires have more mass and softer sidewalls than the closest LT sizes; therefore, more heat is generated. Basically, flotation tires are great for flexing and conforming on uneven terrain – and great for mud – but not so great for hauling and towing.
Is Your 4×4 Over the GVWR?
Most 4×4 owners add weight to their vehicles – especially if they’ve upgraded bumpers, added rock rails, added racks, and then loaded a bunch of camping gear and goodies in their vehicle. If your 4×4 is starting to exceed the gross vehicle weight rating, the very best thing to do is to lighten the load somehow.
If you can’t, than you need to make sure your tires have a load index that’s 20% higher than the rating required by your vehicle’s weight. Also, if you’ve got a pickup loaded with gear, it’s a good idea to compare the weight on the back axle to the weight on the front…you may find that those rear tires are carrying more than the fronts, which means you actually need to increase your load index even more.
Finally, Finding The Right Air Pressure Maximizes Performance and Tire Life
If we’re talking about a Toyota Camry, tire pressure isn’t a big deal. Inflate to whatever the owner’s manual says and move on.
But if we’re talking about a 4×4 with over-sized tires that are intended to haul serious weight – or a 4×4 with flotation tires – there is no “owners manual” you can reference to find the right pressure. Instead, you need to determine the proper pressure so that:
- Your tires will wear evenly and not overheat
- You maintain sufficient pressure for whatever load you’re hauling
While you can “air down” as needed to clear obstacles, you need to keep your tires at the right pressure to ensure performance AND safety. Here’s how:
- Start at a reasonable pressure (the tire manufacturer usually has a standard recommendation)
- Draw a chalk line from one side of the tread wall to the other, going perpendicular to the tread and as straight as possible
- Drive the vehicle around the block, making an equal number of left and right hand turns, and avoiding water. When you return, check the chalk.
- If the chalk has worn evenly, your tire pressure is fine. If the chalk has worn more on the edges than in the middle of the tread, you need to add pressure. If the chalk has worn more on the main tread than on the edges, you need to reduce pressure.
- You can adjust in increments of 2 psi, repeating the test above until you find the right pressure
- Depending on your vehicle, it may be wise to complete this exercise on both a front tire AND a back tire.
Once you establish what your tire pressure should be, it can be fined tuned for towing and hauling. Do this easily by measuring the tire on level ground from the ground to the top of the rim. Adjust the pressure until the measurements of the rear distance from the top of the rim to the ground is 3/16” higher than the front distance. This will ensure the best control and stability on the road. If you can’t reach this measurement, the load may be too heavy, or the the front tires could be over inflated.
Tires Are More Complicated Than They Seem, But They’re Still Pretty Simple
Summing up, there are a lot of specs to look at/think about, but most of the decisions you make when you buy tires should boil down to:
- Are you going to haul and tow, and if so, what is the vehicle going to weigh when fully loaded?
- If you aren’t hauling or towing, but you ARE loading up your vehicle with camping gear and supplies, that’s still considered “hauling” and it’s a smart idea to verify your vehicle’s weight when loaded up.
- Based on your intended use, you want to buy tires that meet your on and off-road needs, and that also exceed your estimated load index by 20%
- Remember that your tire’s load carrying capacity changes with heat, and that things like under-inflation and high speeds increase heat. If you’re going on a long highway trip, make sure your tire pressure is correct, and consider keeping your speed at 55mph or 65mph instead of 75mph, so the tires stay cool.