How Much Do You Know About Your Tires?
By Eric Peters
National Motorists Association
Can you read
a sidewall? What do all those numbers (and letters) really mean?
a short primer on tires and tire-related terminology
that should help you get to know your four friends a bit more intimately:
As your tires
rotate, only a portion of the total tread is actually in contact
with the ground at any given moment. This is known as the contact
patch. Think of it as your tires footprint. Sport/performance-type
tires are characterized by their wider footprint more tread
is in contact with the ground which provides extra grip,
especially during hard acceleration on dry pavement and during high-speed
These are narrow
bands built into the tread during manufacturing that begin to show
when only 1/16 of the tires tread remains. Also called wear
bars, treadwear indicators are there to provide an obvious visual
warning that its time to shop for new tires.
symbol youll find on your tires sidewall that tells
you the maximum sustained speed the tire is capable of safely handling.
An H-rated tire, for example, is built to be safe for continuous
operation at speeds up to 130 mph. Most current model year family-type
cars have S (112 mph) or T (118 mph) speed ratings. High performance
cars often have tires with a V (149 mph) or ZR (in excess of 149
mph) speed rating. A few ultra-performance cars have W (168 mph)
and even Y (186 mph) speed-rated tires. High-speed tires usually
also provide better braking and handling performance, although they
will tend to wear faster, too.
Cold Inflation Load Limit
to the maximum load that can be carried in a given vehicle with
a given type of tires and the maximum air pressure needed
to support that load. In your vehicles owners manual,
you should be able to find the recommended cold inflation load limit.
Its important not to exceed the load limit (or over or under-inflate
the tires) as this can lead to stability/handling problems and even
tire failure. Always check tire pressure cold. Driving
creates friction which creates heat; as the tires warm up, the air
inside expands, increasing the pressure. Measuring air pressure
after driving can give a false reading; you may actually be driving
around on under-inflated tires.
corresponds to the load carrying capacity of the tire. The higher
the number, the higher the load it can safely handle. As an example,
a tire with a load index of 89 can safely handle 1,279 pounds
while a tire with a load rating of 100 can safely handle as much
as 1,764 pounds. Its important to stick with tires that have
at least the same load rating as the tires that came originally
with the vehicle especially if its a truck used to
haul heavy loads or pull a trailer. Its OK to go with a tire
that has a higher load rating than the original tires; just be careful
to avoid tires with a lower load rating than specified for your
vehicle, even if they are less expensive. Saving a few bucks on
tires is not worth risking an accident caused by tire failure.
term refers to the relationship between the width of a tire and
the height of the tires sidewall. High-performance low
profile tires have low aspect ratios meaning
their sidewalls are short relative to their width. This design provides
extra stiffness and thus better high-speed handling and grip
but also tends to result in a firmer (and sometimes, harsh) ride.
Taller tires, on the other hand, tend to provide a smoother
ride and better traction in snow.
have their underlying plies laid at alternate angles less than 90
degrees to the centerline of the tread; radials have their plies
laid at 90 degrees to the centerline of the tread. Thats the
technical difference. The reason radial tires are dominant today
is that they help improve fuel efficiency and handling; they also
tend to dissipate heat better than bias-ply tires. No modern passenger
cars come with bias-ply tires these days and their use is generally
not recommended. (Exception might include older/antique vehicles
that originally came equipped with bias-ply tires. Some RVs also
used bias-ply tires, etc.) It is very important never to mix radial
and bias-ply tires; dangerously erratic handling may result.
LT and MS
indicate Light Truck and Mud/Snow
and are commonly found on tires fitted to 4WD SUVs and pick-ups.
LT-rated tires are more general purpose, built primarily for on-road
use while MS-rated tires typically have more aggressive knobby
tread patterns designed for better off-road traction.
cars come with space-saver tires which are smaller and
lighter than a standard or full-size spare tire. They are designed
to leave more room in the trunk and be easier for the average person
to handle when a roadside tire change becomes necessary. However,
they are for temporary use only and not designed to be used
for extended periods (or high-speed) driving. Your car will probably
not handle (or stop) as well while the space saver tire is on; to
be safe, you should keep your speed under 55 mph and avoid driving
on the tire beyond whats absolutely necessary to find a repair
shop where you can have your damaged tire repaired or replaced.
Traction and Temperature Ratings
Each tire has
three separate ratings for Treadwear, Traction and Temperature.
Traction ratings run from AA to A to B and C with C being
the lowest on the scale. The ratings represent the tires ability
to stop on wet pavement under controlled testing conducted by the
government. C-rated tires are marginal and should be avoided. Never
buy a tire with a Traction rating that isnt at least equal
to the minimum rating specified by the manufacturer of your vehicle.
ratings from A to B to C with C being the minimum allowable
for any passenger car tire. The ratings correspond to a given tires
ability to dissipate heat under load; tires with lower ratings are
more prone to heat-induced failure, especially if driven at high
speeds (or when overloaded). As with Traction ratings, never buy
a tire with a Temperature rating thats less than specified
for your vehicle.
differ from Traction and Temperature ratings in that they arent
a measure of a tires built-in safety margin. Instead, these
ratings represented by a three digit number give you
an idea of the expected useful life of the tire according to government
testing. A tire with a Treadwear rating of 150, for example, can
be expected to last about 1.5 times as long as a tire with a Treadwear
rating of 100. These are just guides, however. Your tires may last
longer (or not) depending on such factors as how you drive, whether
you maintain proper inflation pressure and rotate the tires per
the recommendations and so on.
with permission from the National
Peters is automotive columnist for the National
© 2010 National