Andrew, the big thing that is going to affect your mileage is your right foot. To maximize mileage, you should not drive the truck at a constant speed with an engine RPM over 2200 RPM. Kinda like, if you are accelerating, try to position your foot so the RPMs come up to about 2500 and then the truck shifts to the next gear on its own. Are you going to accelerate slower than the car next to you. Of course. But, what do you want, performance or mileage? Playing costs.
Now, with that being said, there are a few things that you can do to maximize your mileage too.
1) freer flowing intake, whether this is a complete new intake or doing the GOTTS mod, either one makes it so the engine has to work less hard to pull the air in, therefore, that more power is transferred to the wheels
2) open up the exhaust a little bit. Notice I said a little bit. Jumping up to dual 3" pipes with a 40 series muffler is going to be great at 4,000 RPM, but is going to kill any in town mileage due to the loss of back pressure. A nice balance that most seem to like is either running dual 2.5" pipes or a single 3" pipe and then putting something equivalent to a Flowmaster 70 or 50 series muffler on.
3) convert over to electric fan. This is good for 6 hp and 8 ft-lbs of gain. The nice thing about this is you gain it at both 5,000 RPM and at 500 RPM (this is unlike most modifications where you only gain it at the top end). You will find that you will have to get into the gas less to get the same acceleration (ie, the motor is not having to move the fan, therefore that power is transferred to the rear wheels).
4) aftermarket computer. I see this as a double edge sword. Yes, if used in a certain way, you can improve your mileage. But at the same time, you decide you want the power, your mileage is going to tank. If you can control your right foot, it can help. If you can't or like to play a lot, this can be hurtful.
This is a good starting point. I have done a lot of playing around with my truck seeing what works and what doesn't. But then, I am also able to still pull 13 mpg with a lifted truck on 37" mud tires.
