The Physics
Factbook
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Pressure in a Car Tire

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Bibliographic Entry Result
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Result
Tire Pressure, Automobile Repair, Wikibooks, 2007. If suggested pressures are not available, or if the vehicle is modified, Usenet quotes an algorithm from Oscar Pereda, an engineer for BFGoodrich, as a good starting point for pressure in pounds per square inch: (Vehicle Weight in lb/100) psi + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock. [Use the weight of a compact car, about 2000 pounds.]" 138–166 kPa
Carley, Larry. Tire Inflation. 2005. "For many passenger cars and light trucks, the recommended OE tire pressure may range from 28 up to 34 psi." 193–234 kPa
US Court of Appeals, Fifth Circuit. ALMI vs. Firestone Tires. 15 September 1986. "On November 6, 1981, Davis and his helper, Herman Jackson, picked up the new Firestone tire in question from St. Mary Tire & Safety Co. (St. Mary), in Franklin, Louisiana. After Davis and Jackson returned to the Sterling garage with the tire, Jackson, who had been assisting Davis for about one year, tore the red and white inflation warning label from the body of the tire and discarded it before Davis began mounting the tire on the rim. The warning label provided in part:"
"WARNING! NEVER INFLATE ABOVE 35 PSI. USE OF EXCESSIVE PRESSURE DURING MOUNTING MAY CAUSE TIRE/WHEEL ASSEMBLY TO BURST WITH EXCESSIVE FORCE!"
241 kPa
HARMONY tire size and specs. Michelin Tires, 2005. "Only tire professionals should mount tires and they should never inflate beyond 40 psi to seat the beads." 276 kPa

The word car suggests a very broad field. The car category includes the smallest Honda Civic all the way up to a Ford Windstar. A car is assumed to mean an automobile which people use in everyday life for transportation purposes.

As important as cars are, many people neglect a very important aspects of the car; its tires. The tires on a car rotate on the ground without slipping, creating a driving force due to the friction between the tire and the ground.

One thing that is important for proper driving conditions is proper tire inflation and pressure. The ideal range of pressure in cars is between 193-234 in kPa (kilopascal) or 28 to 34 psi (pounds per square inch). However, in heavier cars, such as minivans, the pressure can be higher, upwards to 303 kPa (44 psi).

If a tire is under inflated by 41.4 kPa (6 psi), a loss of handling will be noticed and tire life will decrease. Most notable, fuel economy is reduced by 5%. If over inflated by 41.4 kPa (6 psi), the car will ride harsher since the car is more susceptible to potholes and debris. Worst case scenario, the tire will burst because the pressure is too great.

Conditions also have to be adjusted for faster driving conditions, as shown in the table below for the Harmony tire by Michelin.

SPEED RATED SIZES
Maximum speed (mph) 100 106 112 113
Inflation increase (psi) 0.0 1.0 2.0 3.0
Capacity (% of max) 100 100 100 100

Ultimately, ideal tire pressure depends on the weight of the car. The best thing to do is to read the manufactures suggested pressure which varies by tire brand and make. If you do, then you can be sure that the tire will not "burst with excessive force" and your fuel economy will be improved.

Brian Marino -- 2007