I have the same setup. 08 Scab 5.4 3.73 non limited slip, 18"wheels, towing package. Average 14.5 mpg over 10,500 miles. But the gas where I live has 10% alcohol due to emmissions . If you live ouside a major city, you can get 17mpg with full 100% gas. My friend in upstate NY gets this and he has the same setup as me.