How is the load capacity of a tire determined?

Let's begin by reminding ourselves that it is not the tire that carries the load, but the air inside it. The tire is just the container for that air. So, if you need to carry more load, you need more air.

You might accomplish that with a larger air chamber or by forcing more air molecules into the same size air chamber you'd normally use for a tire with a lower load capacity.

In other words, you might use a bigger tire or a tire running at a higher inflation pressure.

Either way, the tire needs to be strong enough to handle the higher capacity. The traditional ways of defining this are "Ply Rating," "Load Range" and "Load Index."

Bias-ply tires always have an even number of body plies, with the cords laid at angles - on a "bias" - with respect to each other.

What is meant by "Ply Rating"?

In the early days of bias tires, casing strength was built up by adding layer upon layer of cotton fabric. The layers were placed with the thread in each layer at an angle to each other. That added strength, because the tensions would be distributed throughout the layers of fabric.

Tire makers called this positioning of the threads at an angle "bias," and the result is what are termed "bias ply" tires. As you can see from the chart, tires went from two plies to 20 or more.

Why aren't there any odd numbers, 1, 3, 5, 7, etc.?

Since you have to have at least two plies to have a "bias" arrangement, bias ply tires always had an even number of plies.

Is cotton still used?

Cotton went away a long time ago. One of the really big improvements was making plies out of nylon. Nylon is so much stronger that if you're old enough, you may remember seeing your car tires stamped with the words, "2 Ply/4 Ply Rating."

That meant there were only two nylon plies, but they were so strong the tire was equivalent to one made with four cotton plies.

And that's about when things started to get complicated.

How so?

Ply materials continued to improve, especially with the introduction of steel ply materials and radial construction, making the old ply numbers less and less meaningful. And that resulted in the newer designation we use today, called "Load Range."

On the chart, you can see how today's Load Ranges correspond to the older Ply Rating system.

And why isn't there an "I" or a "K" Load Range?

Just to avoid confusion. An "I" might, depending on the typeface, look like the numeral "1," and "K" is a very common abbreviation for "kilo-," meaning "thousand."

What does the Load Range really mean?

Load Range indicates the maximum load recommended for the tire. This varies by tire size and inflation pressure: A bigger tire can hold more air and can be rated for a higher load. Also, a given tire size at a higher air pressure results in a higher rated load.

So what's the difference between tires of the same size but different load ranges?

It's no longer the number of plies. Most Bridgestone radial truck tires, for example, have a total of five plies. There's one steel body ply and four belts under the tread.

What might be different nowadays is the strength of the steel cables in those plies or maybe the number of those cables per inch, or something like that. We're now at the point where we no longer add more and more plies, but instead, adjust the strength of the entire casing to achieve the desired load capacity.

And what is a "Load Index"?

Load Index is another way of rating load capacity, primarily used where the metric system dominates. Load indexes run from about 90 to 170 or more, and are tied to capacities, in kilograms.On a load and inflation table, you'll often see the Load Index just to the right of the loads and the Load Range letters. Load Indexes are not required by the U.S. DOT.

Will a tire with a higher load rating last longer?

It might, but it also might not. What usually determines tire life is the rate at which the tread wears and whether or not the tread wears evenly. Or how many retreads you can get from its casing.

As long as the tire has enough load capacity for the maximum load you will be putting on it, buying extra load capacity may not increase your tire life - or reduce your cost per mile.

As a result, we can't recommend buying, say, "H"-rated tires when "G"-rated tires will do the job.

Which rating system should we use?

The Load Range designations, the alphabet letters, are most common in North America today, though some old-timers hang onto Ply Ratings out of habit. Load Indexes aren't much used here.

Use whichever is easiest for you to keep straight in your head. Bridgestone Data Books publish all three ratings, for your convenience, along with cross-reference conversion charts.