Best gas mileage from cummins dodge diesel?

I have 4 Dodge Trucks. '89 2wd automatic @ 22 mpg (driving normal) and 17 mpg acting stupid with it (pump is turned up). '94 4wd Manual @ 18 mpg (all stock work truck). '96 4wd automatic @ 19 mpg (all stock), '98 24v 4wd automatic 18 mpg (driving normal) and 14.5 mpg acting stupid with it (575 hp and still finding weak links).