I presume by the engine size it is a 96 or newer...and likely an automatic?
Try...
LRR tires, higher inflation, a scan gauge to monitor your driving habits, go slower, avoid stopping, reduce your use of A/C, lighten the load if possible, make sure it is tuned up properly, reprogram your shift points (if it is an auto), check your alignment, check your brakes, and look up "stinkerbutt" <http://www.gassavers.org/garage/view/445> to get some aeromod ideas...kitcar <http://www.gassavers.org/member.php?u=1002> got his 2000 Dakota upwards of 30mpg! Oh...and check the forums.