Before my deletes I was able to get the best mpg by driving more economically and unplugging my EGR. If your willing to throw money at it you could toss in a free spin hub kit and a MPG Max Water/Meth system. Also, always keep an eye on your tire pressure.
Youd have to plug the EGR back in and remove the Water/Meth system before dealer visits to keep warranty though.
If warranty isn't a concern deletes are the best place to start. There are many more advantages to them than just better mpg. Do some searching on this site for what your looking for, there' A LOT of reading ahead of you but a ton of good info.
2010 3500 ST 4X4, Auto, 3.73's, Bedliner, Ride Rites, Holmes 440 wrecker, Removed Intake Silencer, XRT Pro, DPF&Cats gone, H&S Egr/Cooler delete, GDP CCV Breather, 4" TBE to 5" stack straight piped.