Geckoboy your drop in mpg might be that the tires are so new. With new tires I usually run all four at 35 psi cold. After the first rotation at 5000 miles I start to play with the tire pressure. I air them up till they feel uncomfortable usually at 38 psi. I had many sets of tires that went for many mile by using 35 psi.

My TCH came with michelin Energy tires. I read some of the guys here ran 40 to 42 in them. I tried 40 and found they still rode smooth.

Your brand of tires may not be safe or at the pressure. Be sure to check your max pressure on the sidewall.

In the past I bought another brand of tires other than the ones that came on a Chevrolet. I lost 2 and 3 mpg in the process. I aired them a few pounds to help compensate. I read on the tirerack site than oem tires are designed for a smooth ride and to get good gas mileage. If I go to another brand or tire, I try make sure its in the oem list of tires.