You will notice an increase.. Nothing crazy but there will be one. With my 08 I noticed a slight increase with my S&B intake..Nothing jaw dropping but noticiable. My Flowmaster super 44 didn't cause any increase, maybe a loss because I love just hearing it roar. The magnaflows are sopose to be better flowing and may help a small amount. And with my Diablosport Trinity T-1000 I get better miliage when I run it in miliage booster mode. In performance modes I usually have MDS turned off, throttle boosted up a bit and I tend to arrive at destinations alot faster.. Everyone seems to drive slow.. No clue why.
But your biggest factors would be how you drive your truck.. Best advice is to be light on the gas pedal, Use cruise on the hwy at a good speed and not flying by everyone even as tempting as it gets. Even the type of gas you use can play a factor. Certain gas-stations use different grades of gas for the equal octane. I know if I use Esso/ Shell or Petro Canada I get better fuel efficiency then if I use a Pioneer gas station right near my house. Don't sit around idling your truck, if you know your gonna be in the same spot for more then a minute, shut er down. Not using your A/C can help a little too.. I have window visors and always just have the windows cracked on the HWY and fully open when in city.. Engine doesn't have to work as much to power the A/C, even though it only uses about 1HP. When approaching a stop, let off the gas while before and coast/ slow down and then lightly use your brakes to stop.. Sopose to help a bit and also helps with brake life.
With a lift and mudders. You gotta live by the pay to play motto.. Trucks looks awesome, can drive over small cars and go anywhere.. But downside is more wind resistance and road resistance with the tires causing loss of fuel.. But it's a truck, and Rams are pretty good compared to other brands.. If Someone wants fuel efficiency, they get a car.. Not a truck