Results of Earth Day Tire Pressure Project

Picture the roads across the U.S. jammed with 250 million vehicles hurtling along at speeds of 70 mph and faster. Now picture this: most of those cars are riding on dangerously under-inflated tires which waste gas, wear out the tread prematurely and create deadly driving situations.

How under-inflated are those tires? How much gas is wasted?

That's what we wanted to find out as part of an Edmunds.com Earth Day project. We gave out 400 tire gauges to our employees and asked them to check their tire pressure and report it to us.

From the responses of over 212 employees, here's what we found:

• Our employeees' tires were under-inflated by an average of 2.33 psi (pounds per square inch) or 7 percent – far above than the government’s estimate of 26 percent (probably because so many Edmunds drivers are automotive enthusiasts.)
• By filling up tires to the specified amount, our employees will save 5,820 gallons of gas a year (assuming they drive the national average of 15,000 miles a year).
• With properly inflated tires employees will save an average of $112 a year in gasoline (and even more by preventing premature tire wear). Some drivers with severely under inflated tires will save nearly $800 a year.
• If the nearly 250 million registered passenger vehicles in the United States (according to a 2005 Dept. of Transportation study) were only 7 percent under inflated, and brought their tires up to the specified level, together they would save about $23 billion per year. (To learn more about the environmental impact of our findings, check out the Green Car Advisor's comments on tire pressure.)

This project revealed much more than just the level of our employees’ under-inflated tires. We learned a lot about the level of knowledge about tire pressure, inflation levels, tire wear and driving styles.

Many people, even at an enthusiast publication like Edmunds.com, have never checked their tire pressure. This led us to wonder when the tire pressure was ever checked and adjusted in those car’s tires – probably only when the tires are rotated, about once every six months to a year. This isn’t often enough. Tires should be checked and adjusted at least once a month.

We also found that many people don’t know what level to fill their tires to. Many respondents listed the pressure on the sidewall of their tires. This is wrong. The correct tire pressure is listed on a placard found in the car’s door, door jam, glove compartment or in their owner’s manual.

Still more people seemed to be simply guessing how much air to put in their tires. They pumped air in their tires now and then to make sure they were over the required amount. But they didn’t check and adjust the tires so the tires were all approximately at the same level.

Here’s a quick how-to on checking and filling your tires (http://www.edmunds.com/ownership/howto/articles/125093/article.html) .

A week and a half before Earth Day we introduced the tire pressure project at a company meeting and handed out a tire pressure gauge and data form to each employee. On the form we asked that the employee listed their car’s year, make and model, engine size, recommended tire pressure level (as prescribed by the car’s manufacturer), the EPA’s estimated fuel economy and their commute distance. Over the next week and a half, the forms were filled out and returned to us. In some cases, we had to assist people by showing them how to check their tire pressure.

As the forms were returned we entered the data into a spreadsheet, adjusted the fuel economy to the 2008 EPA estimates and created a number of calculations based on the data we gathered. It was a time-consuming process, since we had to look up and convert the fuel economy estimates provided by the EPA (prior to 2008, the EPA estimates were overly optimistic). For cars built after model year 2008, the standards were changed to present a more real world picture of the fuel economy that drivers would actually get.

While current concern about high gas prices has highlighted the fuel economy improvement of properly inflated tires, the most important aspect of this issue is actually safety. A test driver for Edmunds.com said he attended a Michelin tire company event where he drove a course on properly inflated tires and then again on tires under-inflated by only 5 psi. “The result was dramatic,” he said. “But then we did it again with under-inflated tires on a wet course – that was frightening.”

Under-inflated tires don’t allow the tread to make proper contact with the road and the proper grip is lost. This will affect the handling and braking of the car. Under inflated tires will also heat up as you drive and this could eventually lead to a blowout.

Most drivers will not be able to feel that they tires are under inflated by the way the car handles. And under normal cases it won’t significantly affect the driving dynamics of the vehicle. However, if an emergency avoidance maneuver is required, and the tires are under inflated, braking is severely reduces and the car’s handling is compromised. The result could be the difference between life and death.

Our tire pressure survey revealed that many people at Edmunds.com were over-inflating their tires. The assumption must be that if a certain amount of air was required than more air pressure would be even better. This isn’t necessarily true.

When a tire is over inflated the contact patch is reduced and the center of the tire tread wears faster than the outer edges. One tire expert said that this slightly compromises braking distances but actually improves handling because it provides more “bite” as the tire flexes during sharp cornering. This expert said he chooses 4 psi over the specified level.

One argument in favor of slight over inflation is that tire pressure levels diffuse over time. If your normal amount is 4 psi over, and you check it frequently, you don’t run the risk of being under inflated. Furthermore, as the seasons change, and the temperature drops, a slightly over-inflated tire will not drop below the specified amount.

However, in most cases, the best rule of thumb is to check you tires when they are cold, fill them to the level specified by the manufacturer, and recheck them every month.

If tire pressure level has such an important effect on fuel economy, why don’t people take care of this basic task more often?
• In the past, tire pressure gauges were hard to use and difficult to read. Edmunds.com gave its employees digital tire gauges that were easy to use and read. A good digital tire gauge costs about $10.
• The sticker or placard listing the specified tire pressure level can be hard to find and tough to read. If all else fails look for the tire pressure level in the owner’s manual. This is easier on newer cars, as a new government-mandated tire pressure placard was introduced in 2007. It has a standardized shape and design and is always located on the driver’s door jamb.
• Filling the tires at a filling station costs money and often the air pump isn’t working. In many cases, state laws mandate that this service is free. Simply ask the station attendant to turn on the air compressor.
• It’s a dirty job that’s hard to do. With a little bit of practice anyone can learn to use a tire pressure gauge. But you will get your hands a little dirty so find a place to wash up afterwards.

New laws have forced car makers to install tire pressure monitoring systems (TPMS) on all 2008 model cars. However, many of these systems only alert the driver to low tire pressure when the tires are 25 percent low. Our test demonstrates that even a 7 percent loss in tire pressure is costly and dangerous. So it’s important to manually check the tires even on modern cars.

In recent years, because of increased publicity on this topic, and the mandatory installation of TPMS systems on new cars, more people are paying attention to proper tire pressure. Several years ago, the government estimated that tires were an average of 26 percent below specification. Our finding of 7 percent below the proper level in part reflects the growing awareness of low tire pressure as a safety and environmental issue but it also shows that Edmunds employees are perhaps more in tune with such automotive issues than the average driving public. The real average level of tire pressure is probably somewhere between the former average of 26 percent and the Edmunds average of 7 percent.

Keeping tire pressure at the specified level is important for fuel economy, safety and to prevent tire wear. It only takes minutes to check and fill your tires and in most cases it’s free. The only obstacle to achieving this goal is a $10-tire pressure gauge, a little knowledge and initiative. With gas pushing higher every week, and a set of tires easily running $400, it’s high time to make this bit of maintenance a cost-saving habit.

Posted by

Philip Reed April 22, 2008, 4:27 AM

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Fuel Economy