In the event that you aren't able to find a recommendation for your car, or the tires on your car are so different from those that were originally fitted from the manufacturer as to be incomparable, we received the following rule of thumb from Oscar Pereda, an engineer for BFGoodrich. He calls it a "realistic starting point," saying it has never been just right, but is a good place to start. The rule is:
(Vehicle Weight in lb/100) + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock.
Example: Stock 911, 3,000 lb.
(3000/100) = 30 psi
Add 2 psi all around = 32 psi
Add 2 psi to heavy end = 34 psi at rear
With modified suspension, the result is 30 psi front, 32 psi rear.
"There is no 'golden' tire pressure," Oscar said. The optimal setting depends on the individual driver and his preferences. For those inclined to find the ultimate setup for track use, Oscar provided additional instructions. First, get a skidpad, and plan to be dizzy. Take your dramamine. Drive in a circle, first one direction, then the other. Measure and record tire temperature distributions with a probe-type pyrometer that actually penetrates the rubber, not the "aim and click" infrared type. You want bulk temperature, not surface temperature, because the surface cools rapidly while the inner temperature is more stable. When measuring tire temperatures, if center is hotter or cooler than shoulders, there is too much or too little pressure, respectively.
Adjust pressures in 2-psi increments and record all adjustments. Adjust pressures by differences. If you start at 30 psi, and want to take out 2 psi, but the tires have heated up and are at 33 psi, set them at 31 psi rather than 28. The sum of all the changes made will be very close to the change from your initial cold setting. To check this, leave the tires alone at the end of the day, let them cool, and check the pressures in the morning. If at any point during the day you "reset" your pressures to some arbitrary starting place, you are suddenly lost, and all the work you have done that day is gone.
The ideal caster and toe alignment settings will give even tire pressures all across the tread. This can be determined in the same way as the optimum tire pressure. You'll never get the tire temperatures perfectly even, but the best you can do is the best you can do, which is the point of the exercise. What works best will vary from car to car depending on camber curves, body roll, ride height and other factors.
Which tire pressure gauge to use? Oscar told us that, in general, less expensive gauges tend to deviate more at higher pressures. To run the vehicle manufacturer's recommended pressure, accuracy is more important. To dial in a car, precision and repeatability are more important.
For more information, consult your tire manufacturer. The better tire company websites have extensive technical information. Another good source is the Rubber Manufacturer's Association. http://rma.org/tiresafety/
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How to Read a Sidewall
Tire sizing is simple to understand, once you know what the numbers mean. Generally a tire's sidewall will have a set of letters and numbers of the form AAA/BBC-DD EEEF, where AAA = section width, BB = aspect ratio, C = construction, DD = wheel diameter, EEE = load index, and F = speed rating.
Take, for example, a BFGoodrich g-Force T/A KD, listed 205/50ZR-17 89Y. When installed on the designed wheel width, the tire will be nominally 205mm wide at the widest point of the sidewall bulge. It has an aspect ratio of 50, meaning the section height is 50 percent of the section width. Thus, it will be roughly 102.5mm from the bead to the tread. The tire mounts on a 17-in.-diameter wheel. If these numbers were perfectly accurate, the tire would have this rolling diameter: 17+(102.5*2)/25.4 = 25.07 in. BFGoodrich lists the overall diameter as 25.1 in., but many tires of the same nominal size will have slightly different widths or heights.
I have met some enthusiasts who obsessed over the overall diameter when changing tire sizes because of the possible effect on gearing. If a tire starts life with 3/16-in. tread depth and is worn down to 1/16-in. tread depth, it has lost 1/4-in. of its diameter. In the example above, 0.25/25 = 0.01, or 1.0 percent. Thus, I wouldn't worry about overall diameter if it is within plus or minus 1/4-in. of the original equipment.
Returning again to the BFGoodrich example above, the R indicates that the tire is of radial construction. A "Z" in the tire size indicates a Z-rated tire, which means it is safe at least to speeds of 149 mph. The actual speed rating is the last letter, which indicates the upper limit. In our case, it is 186 mph. The load index is the maximum load the tire can carry when fully inflated. In our example, it is 1279 lb.
Speed Ratings
Q = 99 mph, 160km/h
S = 112 mph, 180km/h
T = 118 mph, 190km/h
U = 124 mph, 200km/h
H = 130 mph, 210km/h
V = 149 mph, 240km/h
W = 168 mph, 270km/h
Y = 186 mph, 300km/h
Z = 149 mph, 240km/h and over
In order to create some sort of uniform tire quality grading standard, the federal government has established the creatively named Uniform Tire Quality Grading standards, more commonly known as the UTQG. These standards apply only to passenger car tires, except deep tread, winter or snow tires, temporary use spares and tires with 12-in. or smaller diameters. The grades are for three aspects of tire performance: treadwear, traction and temperature. The grade is molded onto the sidewall of the tire. These grades are good for comparing one tire to another on a basic level, but keep in mind that most high-performance tires will exceed any government minimum standard.
Treadwear is determined on a controlled government test course. It is a comparative scale, meaning that a tire graded 200 would wear 50% better than a tire graded 150. Keep in mind, however, that in the real world, driving habits, road conditions, climate and a host of other factors can make a big difference in what the actual treadwear will be, regardless of the UTQG rating.
Traction grades (AA is best, then A, B and C) represent how well a car is able to stop on wet pavement under controlled conditions. The government tests on both concrete and asphalt, since both have different grip qualities. Note that the government test has only to do with wet braking, not with cornering, acceleration, hydroplaning or any other traction characteristics.
Finally there is the temperature rating, which measures how well the tire resists heat generation and how well it dissipates heat under controlled laboratory tests. Sustained high temperatures can reduce tire life because of the degeneration of materials in the tire. A grade of C means that the tire simply conforms to the safety standard set forth by the government. Grades of B and A are higher levels of performance beyond the minimum required. These tests are done on a tire that is properly inflated and loaded properly. Excessive speed, underinflation or other tire abuse can cause any tire to fail unexpectedly.
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