What all above have said is true. Also remember you're dragging around ~4700 lbs. with an unpowered motor. It's going to take more throttle to get it going, but once moving (such as highway driving) it should do better than what you're getting. In town you may not be that far off, depending on how many times you have to get that mass moving from a stop. If you have a lift, larger tires, racks, off road lights, etc. you'll probably get worse mileage on the highway due to the additional drag created. These things aren't very aerodynamic anyway (Stock drag coefficient ~.41), and any chassis add-ons can make the mileage tank, especially with the smaller motor.
As far as tuners go, none of the "big" name (Superchips or Diablosport) tuner makers have any for your motor. I understand there are a couple tuners out there, but they all seem to get mixed reviews, so you'd have to do a little more research and talk to the folks that use them. Heck, some people don't like the SC or DS tuners for the V-8's either.
06 GCO, Dk Khaki, QDII, Flshing Hi 3rd Bk Lt, MDS "ON" Lt, Airtab Vortex Gens, Debadged, Fumoto Drain Valve, BT Catch Can, Chopped EGR Tube, SRT8 CAI, K&N Filter, Sonnax Trans Line Press Booster, SC Tuner W/Adv Trans Tune App, DS Tuner, Cust tubular hdrs w/3 in Magnaflow/Xlerator AP Exh Sys, Bilsteins, SRT Springs/Sways, Cust Eng Cvr, SRT8 frt/rear bumpers, FRI Sidewinder Cam/6.1 valvetrain, SLP 25% UD Pulley, BBK true 90mm T-Body, SOS Ign Coils w/.060 spark plug gaps, SRT Reps with 265/45&295/40X20's.