I just picked up a 2006 250 quad cab short bed with lariat trim. No add ons. 150K miles +/-. I have run 3 tanks of good diesel and only got 14ish mpg. Bought a Diablo sport tuner (100 on ebay) set it on the 60 HP setting and only gained about .5 mpg on the highway. I drive it like i did my V10 (still in the driveway).
The Kicker is my neighbor has the same truck... Has the Diablo too and claims 18. Another guy that now is in the didge world claimed 18 in his '06... Im dumb founded. Runs great, not jack rabbiting off the line, not towing ( the one tank i did tow on only got 10.2 mixed freeway and in town only 5K lb TT) Why would a great running truck get such low (comparitively)mileage?

 

Why would you think a Tunner that adds Horsepower should give it better MPG. Im thinking the Opposite

Lots of things can contribute to poor MPG so here are some thing to consider/check

Bad thermostat (Diesel truck has no spark plug to ignite fuel mix it relies on Compression & Heat to Ignite) I had bad thermostat running at 173* ECT and down 50 miles per tank fuel changed thermostat got it running at 190* and got the 40-50 miles per tank back

Bad EGR Valve that Hangs open or not working right
or EGR Sytem for that matter
colder air has more Molicules to burn Thus better combustion I Gained 1 mpg maybe a touch more with an EGR Deleate Plus recycling spent air back in wont help it

Clogged Air Filter

Maybe even using Dyno Oil vs Synthetic oil

Brake Caliper that Hangs up

Tire pressure

I have a 6in lift 35in Tires and Average 14-15 mpg City and 16-18MPG HWY Long trip

 

Well, I dont "throw" parts at trucks. I need to find out how to test the particular items you suggest. I think my predator may be able to help with that. If not, scan guageII?

 

Just used the predator, it shows ECT and it was above 160, tires were checked today... in good shape 65 front 75 rear rated at 75 anyway. air filter new 2 months ago.

 

Measure the temp of all of your brake rotors after driving (IR temp gun). Jack it up and see if the wheels spin freely or if they drag.

If your coolant is only getting to around 160, then you definitely need a new thermostat.

See if it has any codes (and a predator probably isn't a good code reader).

Weak FICMs can reduce fuel economy - scangaugeII is best tool to watch that IMO (but the Tech Folder shows how to measure FICM voltage w/ a DVOM).