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Old 06-02-2011, 08:21 AM   #1
miles@tirerack.com
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Tire Tech:

What do the side wall markings mean?

There is a lot of information on the sidewall of a tire. Typically, you'll find UTQG ratings for treadwear, traction and temperature, the size of the tire, the load rating index number with a speed rating index, the construction type (bias or radial), the D.O.T. compliance code, construction details, and of course, the make and model of the tire. On some tires used as original equipment, you may also find a marking that indicates its OE status. Porsche uses an N-0 or N-1 designation, BMW uses a star on some O.E. tires and General Motors uses a "TPC" code. Light Truck tires are sometimes marked with an LT for "Light Truck" before the size, passenger tires are often marked with the letter P for "Passenger" before the size. Passenger tires of the same size with or without the P are virtually interchangeable.

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Air Pressure:
Your tires support the weight of your vehicle, right? Well, they don’t! It’s the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tires are to provide all of the handling, traction and durability of which they are capable.

However, you can't set tire pressure...and then forget about it! Tire pressure has to be checked periodically to assure that the influences of time, changes in ambient temperatures or that a small tread puncture has caused it to change.

The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle’s recommended "cold" tire inflation pressure. This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun’s radiant heat affects it.

Since air is a gas, it expands when heated and contracts when cooled. In most parts of North America, this makes fall and early winter months the most critical times to check inflation pressures...days are getting shorter...ambient temperatures are getting colder...and your tires' inflation pressure is going down!

The rule of thumb is for every 10 Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

In most parts of North America, the difference between average summer and winter temperatures is about -50 Fahrenheit...which results in a potential "loss" of about 5 psi as winter’s temperatures set in. And a 5 psi "loss" is enough to sacrifice handling, traction, and durability!

Additionally, the difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country is about 20 Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle’s tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle’s tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.

And finally, if the vehicle is parked in the sun, the sun’s radiant heat will artificially and temporarily increase tire pressures.

We even evaluated the effects of heat generated by the tire’s flexing during use. We tried to eliminate the variable conditions we might encounter on the road by conducting this test using our "competition tire heat cycling service" that rolls the tires under load against the machine’s rollers to simulate real world driving. We monitored the changes in tire pressure in 5-minute intervals. The test tires were inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the same load, the air pressure in all of the tires went up about 1 psi during every 5 minutes of use for the first 20 minutes of operation. Then the air pressures stabilized, typically gaining no more than 1 psi of additional pressure during the next 20 minutes. This means that even a short drive to inflate your tires will result in tires that will probably be "underinflated" by a few psi the following morning.

Add all of these together, and you can understand why the conditions in which you set your vehicle’s tire pressures are almost as important as the fact that you do set it.

It’s important to remember that your vehicle's recommended tire pressure is its "cold" tire inflation pressure. It should be checked in the morning before you drive more than a few miles, or rising ambient temperatures or sun’s radiant heat affects it.

And by the way, if you live in the North and park in an attached or heated garage you will "lose" pressure when you leave its warmth and venture into the real world outside during winter. Add 1 psi "cold" pressure tire pressure to compensate for each 10 Fahrenheit temperature difference between the temperature in the garage and outside.

Checking your air Pressure:
When vehicle manufacturers select a tire size for a vehicle, they evaluate the vehicle’s gross axle weights, the anticipated use of the tire, and the tire diameter and width. Adjustments to these factors give the manufacturer a way to improve handling and appearance. This is especially true for performance tire sizes. The size selected is rarely limited to only one capability (i.e. carrying the vehicle’s weight). The tire usually needs to have additional load capacity as well. This extra capacity is important because without it all of the tire’s performance would be used up just carrying the weight of the vehicle and little would be left for durability at high speeds or responsive handling. For all vehicles produced since 1968, the original tires sizes and inflation pressures (including the spare) are listed on a vehicle placard. This placard can be located on:

The driver-side door or door jamb (Ford vehicles on the rear passenger door jamb)
Glove box or counsel door
Fuel filler door
The engine compartment
Additionally, some manufacturers also list the original tire pressure in the vehicle’s owner’s manual.If a car’s inflation pressure has varied from that which was recommended by the manufacturer, it’s likely that the tire’s wear and performance characteristics have also changed.

Underinflation

If your vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would find a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.

Overinflation

If your tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races, run higher than normal inflation pressures.

The pressure must be checked with a quality air gauge as the inflation pressure can not be accurately estimated through visual inspection.
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Tire Dimensions:
The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2 (remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

Example...185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"

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Speed Ratings:
It is important to note that speed ratings only apply to tires that have not been damaged, altered, underinflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer’s original speed rating, even after being repaired.

In Europe, where selected highways do not have speed limits and high speed driving is permitted, speed ratings were established to match the speed capability of tires with the top speed capabilities of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at “odd” mile per hour increments). Despite the tire manufacturer’s ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits.

Speed ratings are based on laboratory tests where the tire is pressed (to reflect its required load) against a large diameter metal drum and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire’s required speed has been met.

Speed Rating - Miles/Hour - Kilometers/Hour - Typical Use
N=87 MPH, 140km/h, Spare Tires
P=93 MPH, 150km/h
Q=99 MPH, 160km/h, Winter, LT Tires
R=106 MPH, 170km/h, LT Tires
S=112 MPH, 180km/h
T=118 MPH, 190km/h
U=124 MPH, 200km/h
H=130 MPH, 210km/h, Sport Sedans
V=149 MPH, 240km/h, Sports Cars
Z=149 MPH, 240km/h and over, Sports Cars
W=168 MPH, 270km/h, Exotic Sport Cars
Y=186 MPH, 300km/h, Exotic Sport Cars

*Today, the Z-speed rating is the only speed rating that is still branded “within” the tire size, as in P225/50ZR16. All other speed ratings are shown in the tire’s service description.

When Z-speed rated tires were first introduced, they were thought to reflect the highest speed rating that would ever be required. Since that time the automotive industry has found it necessary to add W- and Y-speed ratings (indicated in the tire’s service description) to identify the tires that meet the needs of new vehicles that have extremely high, top speed capabilities.

While all Z-speed rated tires are capable of speeds of 149 mph and above, prior to the W- and Y-speed ratings were identified in the service, how far above 149 mph was not identified.

Prior to 1991, the most popular speed ratings were “S,” “H” and “V.” However, while the speed capabilities of S- and H-rated tires still indicate the same speeds as before, the V-speed rating has been modified. Previously a V-speed rated tire with the “V” branded “within” the tire size indicated that the tire was capable of 130+ miles per hour. Today’s new V-speed rated tires are always identified in the tires service description

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Tire Rotation:
Tire rotation can be beneficial in several ways. When done at the recommended times, it can preserve balanced handling and traction of the tires and even out tire wear. It can even provide performance advantages. When should tires be rotated ? We recommend that high performance tires be rotated every 3,000 to 5,000 miles, even if they don't show signs of wear. Tire rotation can often be done with oil change intervals while the vehicle is off the ground anyway. Tire rotation helps even out tire wear by allowing each tire to serve in as many of the vehicle's wheel positions as possible. Remember, tire rotation can't correct wear problems due to worn mechanical parts or incorrect inflation pressures. It's also important to check your owner's manual for specific details on what method of tire rotation the vehicle's manufacturer recommends.

While every vehicle is equipped with four tires, usually the tires on the front need to accomplish very different tasks than the rear tires. And the tasks encountered on a front wheel drive car are considerably different than those of a rear wheel drive car. Tire wear experienced on a performance vehicle will usually be more severe than those on a family sedan. Each wheel position can cause different wear rates and different type of tire wear.

While no one likes their tires to wear out, it is actually an advantage when all of the tires on a vehicle wear at the same rate throughout their life. As tire wear reduces tread depth, it allows the tires to respond to the driver's input more quickly and increases dry road performance. Since tire rotation will help all of the vehicle’s tires wear at the same rate, it will keep the tires performing equally on all four corners.

When your tires wear out together you can get a new set of tires, without being forced to buy pairs. If you replace tires in sets you will maintain the original handling balance. And our suppliers are constantly introducing new tires, each of which improves upon their past product's performance. If you replace your tires in sets, it allows you to experience today's technology, instead of being forced to match yesterdays.

On front wheel drive cars, rotate the tires in a forward cross pattern.
On rear wheel or four wheel drive vehicles, rotate the tires in a rearward cross pattern.
If you car has directional wheels or tires, rotate them front to rear.
If you car has non-directional tires that are a different size from front to rear, rotate them across the axle.

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Last edited by miles@tirerack.com; 08-02-2011 at 06:30 PM.
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