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The case of tire pressure versus fuel economy

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By James D. Halderman, Straight talk Updated 1:41 PM Friday, October 23, 2009

Wheels: Mike writes by e-mail: “Read your column about low-rolling-resistance tires. I am under the impression that a tire inflated a couple pounds over the recommended pressure gives better gas mileage. I figure the tire is a little firm and therefore rolls better.

I have a 2006 Chevrolet Impala and keep the tires at 36 pounds of air pressure. I coast as much as possible in the city and on the highway. By doing this, I get around 21 mpg in the city and have gotten up to 37 mpg on a long trip to northern Ohio. I get better gas mpg on hilly roads as I can coast longer and do not put more than necessary speed to gain the uphill pull. I am not a slow driver as I keep up with most traffic in the city, but I do not race up to a light and sit there until it changes. I try to anticipate the light and get there in time for the traffic to move, in instances that I can.

Am I fooling myself, or is there something to having a little more air in my tires ... or do I have too much air in my head? Thank you.”

Halderman: You are correct that over-inflating tires can and will reduce the rolling resistance. However, as you mentioned, the ride quality will suffer and the handling will also be affected, including possible reduced braking distance. This is why I cannot and should not recommend anything except what the vehicle manufacturer recommends. They are looking for fuel economy also but they have to balance this with safety.

If you own a vehicle that has a tire pressure monitoring system (TPMS), the sensors can detect over-inflation as well as under-inflation conditions and could cause the TPMS warning lamp to come on warning of a problem. All vehicles 2008 and newer are required to be equipped with this system; many older vehicles may also as the system was being phased in since 2004.

With cooler temperatures, many owners of vehicles that are equipped with tire pressure monitoring systems may see an amber tire pressure warning light come on, especially in the morning, then go out after the vehicle has been driven a few miles. This happens when the tires cool and the air pressure drops 25 percent (about 8 PSI) below the recommended pressure limit as specified to be the threshold for the warning light to be commanded on. Tire pressure drops about 1 pound per square inch for every 10-degree drop in temperature. When the vehicle is being driven, the flexing of the tires generates heat and the pressure increases slightly often enough to cause the warning light to go out.

If the warning light is on and all tires are properly inflated, check the pressure in the spare tire. If the vehicle is equipped with a full-size spare, it is usually equipped with a pressure sensor.

James D. Halderman is an ASE-certified master technician, a member of the Society of Automotive Engineers and the author of 12 textbooks. Submit your automotive care questions to Halderman via e-mail at jhalderman2@juno.com, or write him in care of: Wheels, Marketing Publications Department, Dayton Daily News, 1611 S. Main St., Dayton OH 45409.

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