It's funny you say that I had to take a trip 3wks ago to Louisville Ky empty and back empty and tried that coming back and it actually cut my milage about .5 mpg .....???? I sure though it would work....There's definitley programmer options that will help a good bit with mileage. But they all pretty much involve removing your DPF and the rest of the stock exhaust system. This in turn will void your warranty. A lot people don't have a problem ditching their warranty, and if you're one of them, then this is the route I'd recommend. You can see anywhere from a 2-5mpg improvement with a tuner and turbo back exhaust system.
We've got several different packages, or I could put together a custom package for you if you're interested
The other thing I'd recommend trying, is using 5th gear on the highway, and locking out 6th (shift into manual mode and select 5) 6th gear and 35's will give you way too low of rpms to get good mileage. Once you get in the sweet spot rpm wise in 6th gear you'll be going to fast to get good mileage. So try 5th for a tank of highway driving and I bet it improves some.
Let me know if you'd like to know more about the programmers. There's some great setups out there that are proven to give you a good mileage increase as well as some extra power.
Honestly my 35's and lift dont cut down my milage too much just a little but nothing that some tuning wont fix
Only 6000 miles? Park the work car and drive the DM !!!! LOL
Ok, seriously, keep up the maintenance. ESPECIALLY keep an eye on the air pressure in your tires. I would also drive it around the block a couple of times a week, so the tires don't get flat spots. Bias plys were bad about it, but radials get flat spots too, just not as bad. Fuel stabilizer. ANY programmer will void the warranty at most dealerships. Even if you are just wanting to use it for gauges, try to convince the dealer of that...Good luck! LOL