Something that might shed a bit of light on the world of treadwear ratings
is how they're set. The government sets the specs and the standards for
DOT acceptance, "traction", "temperature" and "load range" ratings, but
treadwear is a voluntary deal because there are so many variables that
could affect it in the Real World that it's essentially meaningless right
from the word go -- but it's good marketing boo for the guys who want to
buy one set of tires that will take them to their graves -- 20 or 30 years
hence. DOT ratings require a certain minimum amount and distribution of
certain things ("stuff" as Dave Saunders of BFG would say) inside the tire
-- nylon and fiberglass plies, bead construction, sidewall puncture
resistance, etc. The scary thing is that the DOT standards haven't been
updated in a long, long time as far as the minimums go, which is how the
Hoosier AS303/RS303 line gets a DOT rating despite a distinct lack of steel
everywhere but the bead. As far as longevity and pothole resistance goes,
think 1940s or early 1950s levels. But boy do they stick on an autocross
course.
How the tiremakers rate treadwear is to come up with some "reference" tire
-- their choice -- which is assigned an arbitrary rating of 100. To rate a
new tire design, you compare how it performed on an accelerated wear test
with the reference tire's performance on the same test. If a tire is 40%
better than the reference tire on the test, it gets a 140 rating. The fact
that "performance" tires like the Comp T/A ZR have ratings like 250 or 300
should tell you something about the reference tire used for the test.
(Yes, I know the Comp T/A ZR is now 'obsoleted' by the g-Force TA KD and
KDW.)
Since it's a self-referential test, there's absolutely nothing stopping a
company from using, say, the equivalent of a Hoosier AS303 (autocross
compound), or, if they were feeling really wicked, a Hoosier Dirt Stocker
as the "reference" tire, assigning it a treadwear value of 100. Then, it
might *legitimately* give the RS303 (road-race compound) a 140 or better
treadwear rating -- all it has to do is be 40% better than the reference
tire. And the tire company doesn't even have to falsify any test results
-- it can even be independently verified by providing an example of the
"reference" tire for the treadwear test.
And this very thing, of course, is why a treadwear rating is unenforceable.
Even without tire company hijinks currently, I have grave doubts that tire
company A's 140-rated tire will get the same results on the treadwear test
as tire company B, C, D, etc.'s 140-rated tires. Some 140s are more equal
than others -- it all depends on the reference tire the company uses.
Treadwear ratings are (theoretically) only applicable as relative
comparisons between different tires in the same company's line, but there's
nothing that says they can't use a different reference tire for different
lines, like one very soft tire for the performance line, another one
somewhat harder for the Things That Go On Buicks And Grand Marquis line.
Not saying this is done, just saying it's possible.
Jim Crider
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