Don't know that much about 273s (or 360s, for that matter), but I'll reiterate that technology is very good way to improve MPG. My '01 Durango R/T (5.9 - 360, basically) gets 19 MPG on the highway at 60 MPH. Granted, it has a K&N FIPK kit, Fastman TB, headers, and 3" exhaust all the way back (2.5" into a Y-pipe), but it's doable. The rest is factory through the 46RE and 3.92 gears. Full tank of gas and driver comes in at 5240 pounds.