Jeff Garmon at Southern Truck Parts has a 5. 9 12 valve in a 2wd Dakota. It has about 370 hourse to the ground, an HX 40 and no Intercooler. Now get this... . he gets 30. 5 MPG. And that's with playing. I have no idea how much more a 3. 9 would get. However, while there, we discussed the 3. 9, apparently, it shakes like a monster at anything below 1300 RPM or so. I don't know if this can be fixed by the motor mounts, a different balancer or what. Just thought I'd give a lil' heads up.
Josiah