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
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
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
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
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
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.
for tires in dual assemblies are set
lower because the tires may not be loaded
wear can cause unequal loads.
crown may put more load
on the inner tire.
camber can cause
unequal tire loading.
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
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
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
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
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
Metric and Wide Base Tires Tires Rated
|66 thru 70
|51 thru 65
|41 thru 50
|31 thru 40
|21 thru 30
|11 thru 20
|6 thru 10
|2.6 thru 5
|creep thru 2.5
The actual load rating for
a tire also depends on the maximum
speed the vehicle will be traveling
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
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
Tires manufactured after May 2003 must
show maximum load and inflation in both
metric and U.S. units.