New DOT numbers use the last four digits to signify the week and year of Manufacture.

Why did the stampings on tire sidewalls change?

In response to some confusion about the date codes that are a part of D.O.T. numbers, the industry responded with a new system.

How is the new date code different?

The old code had just three digits: the first two represented the week of the year during which the tire was made, and the third digit represented the last digit of the year of manufacture.

But this caused some confusion. With casings lasting longer and longer, it sometimes was hard to figure out whether a last digit of “2” meant 1992 – or 2002.

In some cases, a little triangle was added to the right of that last digit, to indicate the decade. So a tire made in 1992 might have a “2” followed by a little triangle.

Since this required adding an extra symbol anyway, why not just put in a numeral for the decade? And that’s what the new date codes do.

So, a tire made in 2003 will have the first two digits representing the week and the last two will be “03.”

What else is new?

Labeling regarding maximum loads changed as well. You’ll now find both metric and U.S. load and inflation information, as you can see in the picture.

In order to conform to international standards, loads are listed in both kilograms (abbreviated “kg”) and pounds (a kilogram weighs about 2.2 pounds).

Pressures are reported in both psi and metric units called “kilopascals” (one psi equals about 6.9 kilopascals).

Tires manufactured after May 2003 must show maximum load inflation in both metric and U.S. units

There’s something different about the single and dual loads too, isn’t there?

There is. Effective in 1999, the Tire and Rim Association (TRA), which sets standards for load and inflation, decided to change the old system.

In early days, tires didn’t have different loads for single and dual use. Everybody assumed they would be the same. But, over time, manufacturers realized that the actual load on each tire in a dual assembly is rarely the same.

Why is that?

For one thing, the tires may not be equally worn.
If there’s a big difference in wear, there’s a difference in overall diameter.

The larger tire will carry more of the load.

Road crown also affects load. The inner tire of a dual assembly can be more heavily loaded than the outer one.
And, there are factors like camber that can also make the loads shared by the tires in a dual assembly different from each other.

	Loads for tires in dual assemblies are set lower because the tires may not be loaded the same.
Unequal wear can cause unequal loads.
Road crown may put more load on the inner tire.
Excessive camber can cause unequal tire loading.

What’s the result?

TRA decided it was wise to rate dual assembly tires at a lower level than tires used singly. Generally, the dual load rating is about 5 to 10 percent less than the single load rating.

And, in old load and inflation tables, these ratings were often at different inflation pressures. So, when comparing single and dual assemblies, not only were loads different, but inflation was often different too.

That can be a bit confusing.

Exactly. The good news is, since 1999, the maximum single and dual loads are now specified at the same pressure, so when you’re comparing single with dual, you no longer have to remember pressure differences.

Did the loads actually change?

Some didn’t change at all. Some changed a little, either up or down, but not a huge amount. And, these changes also brought load and inflation tables more into line with the “Load Index” system, another international metric standard.

Does the difference between dual and single loads compensate if a tire in a dual assembly goes flat?

Unfortunately not. The second tire in a dual assembly is not a spare. It cannot take up the slack
if you have a flat.

Let’s say you’re loaded to the legal limit, 80,000 pounds, your load is evenly distributed, and you’ve got 18 tires under it.
Assuming the steer tire load is 12,000 pounds, dividing the remaining 68,000 pounds among 8 dual assemblies, puts each one at 8,500 pounds, or 4,250 pounds per tire.

For a 295/75R22.5 G-rated tire, inflated to 110 psi, the maximum load is 5,675 pounds (in a dual configuration 6,175 pounds as a single). So, if one tire goes flat, the other is overloaded by (8,500 – 6,175) = 2,325 pounds. That’s a 38 percent overload.

The remaining tire can’t safely take on the load that was previously on two tires. And, if you run your tires at lower inflation pressures, the overload would be even greater.

What if the driver goes really slowly?

It still doesn’t work. At very slow speeds, the load capacity of a tire can increase. In fact, at a maximum speed of 5 miles per hour, that same, G-rated 295/75R22.5, inflated to 110 psi, can carry a load of 7,786 pounds.

Unfortunately, that’s still 9 percent (over 700 pounds) overloaded. To get to the correct speed for the load, you’d have to go to what is called “Creep,” 200 feet or less in each 30-minute period.

Imagine one of your drivers trying to do that!

Anything else to note about sidewall markings?

One thing hasn’t changed: The ratings molded into the sidewall of the tire represent minimum inflation pressures for maximum loads.

For best overall wear and handling characteristics, you should consult load and inflation tables and set your inflation pressures to handle your maximum actual loads, rather than the maximum allowed by the tire’s load rating.

Tires manufactured after May 2003 must show maximum load and inflation in both metric and U.S. units.