You will actually get BETTER milage out of your tires. You will be riding on more contact patch width, so the center will not wear out as fast.
Good way to tell.......draw a chalk mark across the entire width of your tire at your current PSI. Drive in a straight line for a couple revolutions of the tire. Get out and see where the line is worn off. I am betting it is about 60% of the actual width, right down the center, and leaves the chalk mark a couple inches on the outer edges. Now air down a couple PSI and repeat until the chalk mark is touching all the way across. This will give the best TIRE WEAR. It will have a very slight detriment to the MPG though, like maybe 20 miles less to a full tank?