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The information on this page was gathered as a result of my own
curiosity about automobile wheels and tires. It is all information that
was found on the web with search engines. I don't claim to be any sort
of tire or wheel expert, but all the information listed below should be
verifiable, if you find any mistakes, please let the webmaster
Do with it as you please. If you find other useful bits of wheel and
tire related information, feel free to
pass on a link to be added to the page.
Everything you ever wanted to know about a tire is written on the
A = 2 Ply Rating
B = 4 Ply Rating = 35 psi max. load inflation
C = 6 Ply Rating = 50 psi max. load inflation
D = 8 Ply Rating = 65 psi max. load inflation
E = 10 Ply Rating
= 80 psi max. load
F = 12 Ply Rating
= 95 psi max. load
G = 14 Ply Rating
H = 16 Ply Rating
J = 18 Ply Rating
L = 20 Ply Rating
M = 22 Ply Rating
N = 24 Ply Rating
XL= Extra Load
L78/15 = 30x 9.5
N78/15 = 31x 9.5
P78/15 = 33x10.0
Q78/15 = 36x11.5
Q78/16 = 36x10.5
R78/15 = 37x12.5
R85/16 = 37x12.0
Tires are rated at the maximum, sustained speed they can safely operate
at. The speed rating and load index are
usually listed together on the sidewall in a combined alpha-numeric
marking, such as 82S. For simplicity, the maximum speed is
designated by a letter code as followsL
One thing to note in the above table is that it seems the speed rating
values were likely set up in the metric (KPH) unitsm since they have
nice even incrementsm inllike the odd steps in the MPH column.
Therefore, the MPH values are likely subject to rounding errors of +/-
And for ST Trailer Tires, the
speed rating is 65 MPH, so be sure to watch that towing speed with a
Most tires include something called UTQG ratings. The initials
stand for Uniform Tire Quality Grading, a quality rating system
developed by the Department of Transportation (DOT). The system was
designed to provide information to consumers as to the relative
performance of passenger tires in the areas of tread wear, traction
(wet) and temperature. It applies only to automobile tires with a rim
diameter of 13" and larger, but not for snow tires.
The UTQG designation is made up of three attributes of the tire, namely
tread wear, traction, and temperature resistance. An example would be:
A tire with known wear data is used as a control tire, and the results
are corrected for a possible shift in testing conditions compared with
the point of departure. A tire with marking 160 gives 60% more mileage
than a tire marked 100.
The test should be performed with a special measuring vehicle where the
tire is braked with locked wheels on wet asphalt and wet concrete road
A tire with value C, therefore, has the poorest wet traction.
The tire to be tested is inflated to 24psi (165KPa) and installed on
the test apparatus (instrumented trailer). The tire is loaded to 1,085
pounds (492kg). The trailer is towed over the wetted test area at 40mph
(65km/h) and the rotating wheel is locked. The tire is dragged in this
locked condition through the test area and the friction created is
measured. From these measurements the friction efficiency index of a
tire can be calculated using the following formula.
The temperature grades represent the tire's resistance to the
generation of heat when tested under controlled conditions on a
specified indoor laboratory test wheel.
The temperature test protocol consists of running the tire at a series
of test speeds for given time intervals. The speed at which the tire
exceeds the maximum test temperature is used to determine its
temperature grade letter:
It should be noted that the test speed may exceed the speed rating of the tire which explains why
tires may have different temperature ratings.
Another marking found on tires sold in the US includes the Department
Of Transportation (or DOT) marking. Essentially the DOT marking serves
as the tire's fingerprint. DOT signifies that the tire complies with
U.S. Department of Transportation Tire Safety Standards, and is
permitted for highway use.
The load index refers to the load carrying capacity of a tire, or how
much weight a tire can support. For example, if a tire has a load index
of 89, it can support 1,279 pounds (from chart, below) at maximum air
pressure. Multiply that by four (4 x 1,279 = 5,116 pounds) to get your
maximum load carrying capacity. More correctly, you should take into
account the front to rear weight distribution of the vehicle. The load
index and speed rating are usually listed in
a single alpha-numeric code such as 82S.
Note: It is not recommended to install tires with a lower load index
than what came on your car from the factory.
Essentially, these are two similar ratings for the tire's load carrying
So how exactly does a pneumatic tires work and what is the purpose of
A tire is a pneumatic system which supports a vehicle's load. It does
this by using a compressed gas (usually air) inside to create tension
in the carcass plies. It is important to realize that a tire carcass
has a high tension strength, but has little or no compression strength.
It is the air pressure that creates tension in the carcass and allows
the tire to function as a load-carrying device. That's why inflation is
so important. In an unloaded tire, the cords pull equally on the bead
wire all around the tire.
When a tire is loaded, the tension in the cords between the rim and the
ground is relieved by pressure from the ground. The tension in other
cords is not changed. Therefore, the cords opposite the ground pull
upwards on the bead. This is the mechanism that transmits the pressure
from the ground to the rim.
However, a tire's job is more than to hold a load. It must transmit
handling (acceleration, braking, cornering) to the road. Cornering
forces are transmitted to the rim in a similar manner to load.
Acceleration and braking forces rely on the friction between the rim
and the bead. Inflation pressure also supplies the clamping force which
creates this friction.
A tire also acts as a spring between the rim and the road. This spring
characteristic is very important to the vehicle's ride. Too high an
inflation pressure causes the tire to transmit shock loads to the
suspension and reduces a tire's ability to withstand road impacts. Too
low an inflation pressure reduces a tire's ability to support the
vehicle's load and transmit cornering, braking, and acceleration
forces. Finding the optimum inflation pressure requires extensive
engineering efforts on the part of tire and vehicle manufacturers.
Under-inflation can cause many tire related problems. Since a tire's
load capacity is largely determined by its inflation pressure,
under-inflation results in an overloaded tire. An under-inflated tire
operates at high deflection resulting in decreased fuel economy,
sluggish handling and excessive shoulder wear. High deflection also
causes excessive heat buildup leading to catastrophic tire failure.
There are several issues dealing with tire inflation. Among them are
proper inflation for both on-road and off-road use. I've listed some
various "formulas" and techniques to use to determine an
appropriate tire pressure for a given application. You may find you get
conflicting answers, some are more correct and some are less correct.
Every vehicle should have a manufacturer recommended tire inflation
value, usually on a sticker on the driver's side door jamb. This figure
is determined by the manufacturer based upon the vehicles stock weight
distribution, wheel and tire size. This is probably the best value to
use if it applies. However, if you have changed wheels, tires, or
weight significantly, this number may not be appropriate. Also, it is
sometimes unclear as to what were the assumptions used to determine
those pressures. For example on a pickup or SUV, they often list a much
higher rear than front tire pressure. This is likely a pressure setting
at the maximum vehicle load ratingm so on a 1/2 ton pickup, for
example, thise would be with approx. 1000 lbs. or cargo in the bed.
With little or no bed load, this "factory" tire pressure may
be too high, so take those numbers with a grain of salt.
Tires, too, come with manufacturer-specified inflation specifications.
These, however, are not vehicle specific, but rather refer to the
maximum inflation pressure the tire can handle in relation to its
maximum load carrying capacity. For example, assume you have a light
truck tire with a 2500 pound maximum load rating at 50 PSI air
pressure. Lets say there are four of these tires mounted on a 5000
pound vehicle (with 50/50 weight distribution), so the per-tire load is
1250 pounds (5000/4). Clearly, the tire is nowhere near its maximum
load, in fact it is at 1/2 load in this case. A case could be made for
inflating the tire to 1/2 its maximum pressure (25 PSI in this case)
based upon the load on the tire.
Actually, while there is a fairly linear relationship between a tire's
inflation pressure and its load carrying capacity, it is simply not a
straight line from 0 to the maximum load. I did a least-squares-fit
analysis on some pressure vs. load data for a series of agricultural
tires and found that the following factors seem to fit the data quite
mL = maximum tire Load (lbs)
mI = maximum tire Inflation (psi)
L = the actual load on the tire (lbs)
L = 0.21*mL + (0.79*mL/mI)*inflation
In other words, at "0" psi, a tire ideally could carry 21% of
its maximum load (probably not true - but useful for numerical
analysis) and the other 79% of its load capacity is linearly related to
its internal pressure. So, from the example above:
mL = 2500
L = 1250; solve for inflation = (1250 - (0.21*2500)) /
(0.79*2500/50) = (1250-525)/39.5 = 18.3psi
So, 18 is clearly less than 25 (that was obtained with a linear
interpolation), the "correct" answer is probably somewhere in
between. I can say that I have run extended periods on these tires at
18 psi on pavement at highway speeds without any adverse affects. In
fact, before going through these calculations, I had settled on 18psi
as my air-down pressure for off-roading where significant periods of
high-speed/pavement driving was anticipated. This pressure allowed
decent off-road traction and let me safely cover paved sections without
stopping to air-up. So I sort of see this number as a minimum safe
on-road [pressure for extended driving (for these tires on this
Another school of thought is that you should inflate the tire such that
it has uniform tread contact with the road. This can be determined in a
number of ways. The easiest is to try to slide a thin card under the
edge of the tread. Inflate the tire until you can just get the card
under the edge a little bit. A more involved check is to place a chalk
line across the tread face, drive a short distance straight ahead on a
smooth surface and then observe the chalk line. You are looking for it
to be evenly worn off the tread. Another variation is to measure the
length of the contact patch and make it even front and rear. This works
well on vehicle where the rear load can vary, such as a pickup and
especially if a recommended pressure is known for the front end. Slip a
paper sheet under the tire to stop at the leading and trailing edge of
the contact patch, measure the separation of the two sheets (making
sure they are parallel). Then set the rear pressure such that the
length of its contact patch is the same as the front.
One of the most accurate (and complicated) methods is to measure tread
temperatures right after a high speed run. Even temps. across the tread
indicate proper inflation. This is how race teams judge tire pressure
in their vehicles. A less complicated version of this temperature-base
technique is to select a cold tire pressure such that after 15-30
minutes of high speed driving results in a pressure (or temperature)
rise of less than 10%. Increased temperature of the air in the tire is
the cause of the pressure rise, and a rise in pressure of 3 psi is
about 10% of a typical tire inflation pressure (~30 psi) and represents
about a 50°F temperature rise. Note that this represents about a
10% temperature change on a absolute scale, noting that absolute 0 is
-459°F, so at an air temperature of 41°F, the absolute
temperature is 500°F above absolute 0. Thus a temperature change
of 50°F is a 10% increase, from 500°F above absolute 0 to
550°F above absolute 0.
Often when you have new tires installed, you'll find that the tire shop
has inflated them to their maximum rated pressure. Depending on the
vehicle, wheel and tire, this may be good or bad. On a recent set of
tires, I decided to conduct an experiment by collecting tread wear data
over the life of the tires and I changed the inflation pressure about
1/2 way through the tread life. Here is some data I collected over the
life of my Goodyear Invicta GLR tires (175-70R13 mounted on 13x5 rims).
I measured tread depths at the inner, middle and outer tread grooves,
at each tire rotation, taking the average of all 4 tires:
So, looking at the above data, it would appear, that even running these
tires at their maximum inflation pressure, the edges seem to wear
faster then the middle. The tires in this study were rated at 1036 lbs
maximum load, I estimate the VW pickup they are mounted on to weigh
approx. 2500 lbs, so I am well under the maximum load on the tires.
They are mounted on the tire mfg's design width rims, the section width
of the tire is 2" over the rim width. Perhaps different tire and
rim widths as well as different loading characteristics would change
the results. I do drive these tires pretty hard and it looks like I
have worn them out in about 25k-30k miles, but what can you expect from
an OEM tire? Also, they were used (unknown mileage) when I bought them.
They are only rated at 260-tread wear, so I'm not
surprised. One interesting item to note, is that normal passenger car
tires (standard load) are rated at 36psi and maximum inflations above
that value are for meeting special vehicle requirements. There are also
"extra load" passenger car tires for higher load capacities,
that allow higher maximum inflation pressures (44 psi in my case).
So, answering the question of what is the "correct" tire
pressure is not that easy. There are several DIY tests that you can try:
1. The "Business Card Test": On a smooth, hard surface, try
inserting a business card between the tire and the pavement. If it goes
in less than about 3mm-1/8", the the tire may be under-inflated,
if it goes in more than about 6mm-1/4", it may be overinflated.
2. The "Chalk Line Test": Draw a heavy chalk line across all
the tread faces. drive slowly forward in a straight line for a few
revolutions of the tire. Get out and observe the wear pattern of the
chalk. If it has worn away evenly, then the inflation is correct. If
either the edge or center of the line is worn first, then the tire is
under or over inflated, respectively.
3. The "Water Puddle Test": Similar to test #2, but drive
through a puddle of water in a straight line, then get out and observe
the wet tire tracks and see if the wet imprint is even, especially as
the track starts to dry out after a few revolutions.
4. Heat is the #1 enemy of high-speed tires. The flexing of the tire's
sidewalls as the tire rolls under load is the source of the heat.
Higher inflation pressures mean less flexing of the sidewall and
therefore less heat. Another test for proper inflation pressure is to
measure the tire pressure when cold then again after 15 minutes at
highway speed. If the pressure rise due to the temperature rise is more
than about 3 psi, then the tire may be under-inflated.
And some tire manufacturers recommend maintaining a minimum of 25 psi
in on-road tires for adequate bead retention in cornering. Often, you
can contact the tire manufacturer and they can supply inflation data
for your vehicle/tire combination.
Just like on-road, there are several schools of thought on choosing the
correct off-road tire pressure. Off-road, there are many more
variables, such as the type of terrain, the tire and wheel construction
which determine the type of problem you are trying to solve. The
following solutions should work for 15 and 16" rims with safety
beads. Note many 16.5" rims lack safety beads and running lowered
pressures is risky. Rims with bead locks are an entirely different
Anyway, why do you want to lower your tire pressure off-road? Several
reasons come to mind:
So, how do you go about picking a pressure to run off-road?
In any event, you want to pick a pressure that is low enough to handle
the terrain, but high enough to protect the wheel and tire as well as
preventing the loss of the bead.
Now for some terrain-specific observations:
Anyway, I think of tire pressure kind of like cross country ski wax.
You have to know your tires and vehicle, read the terrain and then
choose an air pressure to run. If you err on the high side, you can
always go lower if needed. Of course if you have on-board air, its no
problem either way.
Aside from being inflated for carrying loads, tires need to be round to
work properly. Wheels also need to be round. However, due to
manufacturing constraints, neither may be, exactly. So, wheel
manufacturers tend to place the valve core hole (or some other mark) at
the lowest point of the wheel. Tire manufacturers, on the other hand,
tend to place a mark, usually a red dot, on the wheel's highest pint.
The hope is if you put the highest point of the tire on the lowest
point of the wheel, they will cancel out and you'll end up with a round
Chances are these high/low points won't cancel exactly, how close does
a wheel/tire have to be to be considered round? Generally, passenger
cars are designed to tolerate 0.030" radial or lateral run-out.
Trucks and SUV type vehicles (with larger tires) can usually handle
0.060" radial and lateral run-out. Any more than this and it may
be possible to re-mount the tire in a different position on the rim or
place it on another wheel. If the run-out is not within spec, that tire
should probably be replaced. (I didn't know about this and had a pair
of bias ply tires that had 0.250-0.375" radial run-out - they were
not fun to drive at speed).
Now you have a round wheel/tire, but it needs to be balanced. How close
to perfect can you expect the balance to be? The general guideline is
within 0.25 oz.
See the section on Mounting and Balancing, below,
for details on the interaction of wheels and tires in regards to
A pneumatic tire without a wheel to mount it on isn't a very useful
thing. The wheel provides a number of useful functions, including
attaching the tire to the vehicle at the hub, and for tubeless tire, it
provided the air-tight seal for the inner circumference of the tire.
Wheels have a measuring system that is at first confusing.
Interestingly, it remains almost exclusively in the inch system.
In the above drawings, you can see a typical wheel with important
dimensions shown. Most of the measurements are best thought of from the
perspective of the tire. The rim width is the measured at the point
where the bead of the tire contacts the wheel, same with the rim
If upgrading wheels and/or tires, its relatively easy to determine what
affects the various dimensions will have on the tire location with
respect to the vehicle compared to the current wheels and tires:
As an example, take a vehicle running 31x10.50 tires on 6" wide
rims with 3.5" backspacing. It the wheels/tires were change to
33x12.50 tires on 8" wide rims with 2.5" backspacing, you
would see the following changes in track width:
So, one might check the tire clearance by making up a block of wood
that is 1" tall and 4" wide then move this block around the
outside edge of the current tire under various combinations of
suspension travel and steering input to check for possible tire/body
(and frame) contact.
Finally, wheels will have one (or more) hole(s) drilled in them for the
valve stem. Usually the hole for the valve stem is placed at the lowest
point on the wheel after it has been manufactured and tested. One other
option is that a "dimple" will be embossed on the wheel to
indicate the low point. Why this is important will be seen in the next
On a new set of tires, you'll probably observe various things,
including a sticker on the tread and various dots painted on the
sidewall. You may ask yourself, what are those dots on the sidewall
for? The number and color of the dots may vary by manufacturer, but
here is what Yokohama uses, as an example:
The RED dot indicates the high spot on
the tire and is to be used as the location for measuring tread wear.
"When the indicators show, tires must be replaced."
The YELLOW dot indicates the
lightest part of the tire, also known as "maximum force
variation." This should be lined up with the heaviest part of the
wheel - the valve stem. They call this "phase aligning" the
Actually there are two options for mounting a tire on a wheel:
When new tires are made, there is a mold release agent applied to the
surface of the tire press that allows the cured rubber to release from
the mold pattern. This release agent is, by its very nature, very
slippery and accounts for the "greasy" feel of a new tire.
Its generally recommended to drive approx. 500 easy miles to allow the
release agent (and impregnated surface rubber) to wear away, along with
the extra rubber bits left from the molding process.
Tire Rack has a good article on Breaking
In New Tires.
Below are links to tire manufacturers web pages I've found (and I know
there are more):
Here is some common tire terminology:
[Last updated: 16.August.2012]