Mileage Question

A bit is going to depend on what year Duster and what size /6 you have. Also, what differential gear do you have (a tag on the cover or build sheet decode will tell you.).

Beyond the suggestions already given, suggest checking out SlantSix.org and do a search on "hypermiling driving techniques". Some of the techniques will be too extreme/unsafe. However, there are others such as "timing lights" and coasting that are very good. The name of the game in good city mileage is to safely avoid situations that cause you to alter speed.