http://www.theepochtimes.com/news/6-12-1/48862.html

Going by the rough 'Rules of Thumb' that

A 10% decrease in tire rolling resistance results in a 4% improvement in mixed MPG.

A 10% decrease in aerodynamic drag results in a 5% improvement in highway MPG.

A 10% decrease in weight results in a 6% improvement in city driving MPG.

sample quote:

"According to Dan Guiney, Technical Manager for Yokohama Tire Corporation, which manufactures everything from ultra-high performance tires for passenger cars and SUVs to tires for buses, trucks and airplanes, "Tires that are under-inflated by 6 to 7 psi (pounds per square inch) increase tire rolling resistance 10% or more, increase tread wear rates and tire fatigue."