Visitor # 184859 since 28.AUG.2001
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 know.
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 sidewall.
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 inflation
F = 12 Ply Rating
= 95 psi max. load inflation
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
REIN= Reinforced
A =
B =
C =
D =
E =
F =
G =
H =
I =
J =
K =
L78/15 = 30x 9.5
N78/15 = 31x 9.5
O =
P78/15 = 33x10.0
Q78/15 = 36x11.5
Q78/16 = 36x10.5
R78/15 = 37x12.5
R85/16 = 37x12.0
S =
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 +/- 1 MPH.
And for ST Trailer Tires, the speed rating is 65 MPH, so be sure to watch that towing speed with a trailer.
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 surfaces.
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 capability.
So how exactly does a pneumatic tires work and what is the purpose of the air?
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 well:
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 mI =50 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 vehicle, YMMV:).
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 issue.
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 wheel/tire combo.
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 balancing.
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.
(1) (2)
(3) (4) (5)
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 diameter.
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 section.
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 tire.
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:
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[Last updated: 16.August.2012]
