ask the DOCTOR
The Tire Doctor Responds: Reading and using load and inflation tables really isn’t difficult – once you know how – and once you understand why they’re set up the way they are. In the next few pages,
we hope to take the mystery out of choosing the correct inflation pressure, and show you just how easy it can be.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Where do we start?
Let’s start with the fundamentals: it’s not the tire that supports the load, but the air inside it.
Since we need to know how much air to put into the tire, the first thing we need to know is the load the air has to support. And we need to know it for each tire.

Do we have to weigh each axle to determine the correct inflation pressure?
While that would be the best way, we’re not going to suggest you go to that much trouble. Instead, you can make some reasonable assumptions, then choose inflation based on load and inflation tables.

What are these assumptions?
If we look at a typical configuration, we can tell what typical axle weights might be.

 

 

 

 

 

 

 

 

 

Is that all we need?
We also need to know the tire size and type. For this example, we’re going to assume that all tires are 295/75R22.5 tubeless radials.
Looking at the load and inflation table, we see that for this size, we have two different sets of ratings, one for single and one for dual fitments.
For the steer tires, all we have to do is slide across the “SINGLE” row until we find the first entry equal to (or larger than) our maximum load. As you can see, that’s 6,175 lb.

 

 

 

With a single trailer and tandem drive and trailer axles (and a gross combined vehicle weight of 80,000 lb), we would expect a load of about 6,000 lb. on each steer tire, and 4,250 lb. on each of the other tires.

 

 

 

 

 

While modern radial tires no longer have the numerous plies that were common with earlier, bias tires, here are some equivalents.

 

How does that relate to “Ply rating”?
Ply ratings date back to the days when tires had bias construction and multiple pairs of cotton plies. A modern radial tire may only have four belts and a single casing ply – a total of five in all – but can have a load range equivalent to many, many plies.

 

 

 

And the correct inflation pressure is?
Look up at the top of the chart. You’ll see that the minimum pressure for this load, this size and this load rating is 110 psi.

How about for the other tires?
This time, we must look at the “DUAL” row, and we see that the second entry is equal to or greater than the maximum load, 4,250 lb. At the top, we find 75 psi, the minimum inflation pressure for this load with this tire size and load rating.

How do we know the correct load range for the tire?
Since the entry we found is not boldface, look to the right until you come to a boldface entry. See it? It’s 5,070 lb. and it’s marked “(F).” That’s the load range for the entry we found.
If an entry isn’t in boldface, the load range will be in the next boldface entry to the right of it.

Why are most of the “DUAL” loads lower than the “SINGLE” loads, when the inflation pressure is the same?
On a dual assembly, the two tires aren’t always perfectly matched. Sometimes, one has a slightly larger diameter, perhaps because it has less wear. Or, one might have a slightly higher inflation pressure.
The tire that’s larger, or that has higher inflation pressure, is probably carrying more of the load. Even road crown can put more load on one of your duals than on the other.
So, in a dual assembly, the load limit is reduced slightly, to compensate for small differences in tire loading and inflation pressure.

 

 

 

 

 

 

 

 

 

SOUTH
NORTH
Tires in dual assemblies have lower load limits at a given inflation than single tires to compensate for differences in tire loading as a result of unequal diameters, inflation pressures – or even road crown effects, which differs in different parts of the country.

 

 

 

Are these the ideal inflation pressures?
These are the minimum pressures. You may use higher pressures (that do not exceed the maximum allowable for the tire or wheel).

Why would we do that?
For one thing, we’ve never seen a single documented case where excess air sneaked into a tire. So, if you’re at the minimum, lost inflation pressure is a problem. And you can lose 2 psi per month on truck tires – just through diffusion of air molecules through the rubber.
So, if you start out a little higher than the minimum, you’ve got some cushion against losses.
And, you may find that in your operation, handling, irregular wear – even fuel economy – may improve with a little higher inflation pressure.

So the load and inflation tables are just a starting point?
Yes. Though you should never use pressures lower than what is shown in the charts, you may need to adjust them upward for best overall tire life and performance. Your tire supplier can help you with details.