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Tires

Determining the Right PSI

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More than a few of us have spent time looking for the placard with a vehicle’s tire pressure recommendations. And more than a few consumers have looked at their owner’s manual or that elusive placard and wondered why the pressures there are different from the maximum pressure on the tire’s sidewalls.

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How do the automobile manufacturers and their tire suppliers come up with those numbers?

It starts with the load the vehicle is designed to carry and the size of the tires installed on it. Worldwide, several organizations exist with the primary purpose of establishing and publishing interchangeability standards for tires, wheels, valves and allied parts.carplacard

In the U.S., this standardizing body is the Tire and Rim Association (TRA). Its European equivalent is the European Tire and Rim Technical Organization (ETRTO). The Japanese Automotive Tire Manufacturers Association (JATMA) has that respon-sibility in Japan. There are other similar organizations, but those three are the major players.

None of the major standardizing organizations is government mandated or regulatory. The organizations, and the standards that they set, are all voluntary. There is no legal obligation for any tire or auto manufacturer to abide by the standards. However, for practical and legal liability purposes, it simply makes sense to follow the standards and all vehicle and tire manufacturers use them.

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While TRA, ERTO and JATMA all use slightly different protocols, they consult with each other and their standards are all very close. These organizations establish a range of standard dimensions for each tire size so that the proportions of all tires of a particular size are very similar. This assures that all tires of a specific size are interchangeable, at least in terms of their physical dimensions.

They also set each tire’s “load curve,” which is the relationship between inflation pressure and the tire’s maximum load carrying capacity. Like the size standards, there are some slight variations among the different organizations’ load curves, but they are not significant.

Part of each load curve standard is the point at which the load curve stops going up. For TRA p-metric standard load tires, it is at 35 psi. Equivalent ETRTO and JATMA tires peak at 36 psi. Interestingly, all of the standardizing organizations agree that standard load passenger car tires can (and should) use optional higher inflation pressures (44 psi or 51 psi) for certain circumstances such as high speed driving. However, these higher inflation pressures generally provide no increase in load carrying capacity and in some circumstances specify a reduction in load for high-speed use.

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So the auto manufacturer knows what a vehicle weighs and knows how much weight can be added in fuel, passengers, cargo, etc. They can look at the load curve of the particular size that they have selected. To this number, auto manufacturers usually add a safety factor of around 10%.

The Ford-Firestone Factor

For passenger cars, SUVs and light trucks, the load curve number launches a series of negotiations. The engineers responsible for the vehicle’s handling are likely to lobby for additional inflation pressure to improve handling. Similarly, lower rolling resistance results in better fuel economy and leads to demands for higher inflations. Cars likely to see high-speed bring with them engineers insisting on more air pressure. On the other hand, ride considerations are an important consideration and that calls for lower inflation pressures.

Today, the results of these nego-tiations are inflation pressures typically in the 32 psi to 35 psi range. If the champions of improved ride had their way, the 26 psi to 28 psi range that was common a few years ago would be more common. The safety factor of the higher inflation pressure seems to be carrying the day with automakers. After all, no one wants a repeat of the fiasco of when Ford set the recommended pressures on first generation Explorers at 26 psi.

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In that situation, recommended inflation pressures too low to provide an adequate safety factor were frequently combined with poor maintenance, overloading and high ambient temperatures to produce a series of often-deadly accidents (250 deaths and 3,000 serious injuries were attributed to the problem).

Placard Placement

Other factors keep the process of determining recommended pressures from being completely cut and dried.

Aesthetics is often a big factor when automakers choose tire sizes, and the emphasis on style versus substance can complicate the negotiations. The popularity of extreme low-profile tires has required significant increases in recommended pressures. Cars likely to see significant high-speed often force auto manufacturers to lower load limits, move to higher tire speed ratings such as W or Y, or go to recommendations of up to 51 psi.

Although the process starts with load and seems like it would be simple, nothing is etched in stone.

The recommended pressures go onto the vehicle’s tire information placard once the engineers make their final decisions. Beginning in 2003, federal regulations required automobile manufacturers to place tire information placards in a standardized location and follow a common format. The tire information placards identify the OE tire sizes and inflation pressures (including the spare), along with the vehicle weight capacity.

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All new vehicles must now have a placard located on the driver’s side doorjamb (the B-pillar). If a vehicle does not have a B-pillar, then the placard is to be placed on the rear edge of the driver’s door. And if the vehicle does not have a B-pillar and the driver’s door edge is too narrow, the placard is to be affixed on an inward facing surface next to the driver’s seating position.

In addition to providing the OE tire size(s) and recommended inflation pressure(s), vehicle manufacturers must also identify vehicle load capacity with the following sentence: “The combined weight of occupants and cargo should never exceed XXXX pounds.”

For vehicles produced between 1968 and 2003, the original tires size(s) and inflation pressures were listed on a placard usually found on the driver-side door or doorjamb, rear passenger doorjamb, fuel filler door, glove box or center console door or inside the engine compartment.

Sometimes alternate pressures based on load or speed conditions are also provided.

The Sidewall Says

Consumers are often confused because federal safety regulations also require tire sidewalls to contain information about a tire’s maximum load capacity and maximum inflation pressure. For P-metric standard load tires, the maximum pressure listed on the sidewall will be 44 psi or 51 psi, even though the pressure where the maximum load occurs is 35 psi or 36 psi. On the sidewall, this would look like “Maximum Load XXXX lbs. Maximum Pressure YY psi.”

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A few manufacturers interpret the regulation to mean the maximum load and the corresponding inflation pressure (“Maximum Load XXXX lbs. at YY psi”). Either way, too often consumers see the maximum inflation pressure on the sidewall and believe that is the correct pressure for their car.

Look at some of the online forums and you will see that the arguments about relying on the placard or the tire sidewall can be animated.

Regardless of the information on the tire sidewall, assuming the right size tire is installed on the vehicle the placard is the correct information.

While the maximum load of the vehicle is the primary factor in establishing the recommended pressures, many other considerations went into the calculations. Don’t make changes lightly. Varying from the vehicle manufacturer’s recommendations requires careful consideration of why you want to make the change and the possible safety implications.

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