http://money.cnn.com/news/newsfeeds/...08112007-1.htm

sample quote:

"Running a tire 20 percent underinflated - only 5 to 7 pounds per square inch (psi) - can increase fuel consumption by 10 percent. That can easily cost motorists two or three miles per gallon. Not only that, but running underinflated also reduces the tire's tread life," said Bob Toth, Goodyear's general manager, auto tires. If that's not enough, the Society of Automotive Engineers reports that 87 percent of all flat tires have a history of underinflation.