Rear Gears for gas mileage

That cam you have in it is probably the biggest reason for bad fuel mileage. It's designed to idle like a race car so it isn't very efficient. If you want it to thump expect bad fuel mileage. Tuning, as 72BlunBlu said can make a big difference too. If it were me I'd put in a decent cam that'd actually make good power and idle a little lumpy, something like a Lunati 60403. I ran the next size cam bigger than that in a 360 with 3.55 gears and got 14 mpg.