Every little bit helps for better gas mileage all these can contribute

Wheel bearings
Ball joints
Dragging brakes kind of common for a Dakota
A/C compressor bearings going bad
Power steering pump going bad
Clutch slipping or auto trans slipping
Rear axle wheel bearings worn out
Rear diff problems
Poor quality oils used engine, trans and diff
Large tires wrong size for gearing of rear diff and trans
Need tune-up
Engine worn out low compression
Emission system out of whack bad O2, TPS, MAP, OTS vacuum leaks so on
Old CAT or muffler bad, constrictive exhaust pipes
Throttle body getting clogged up with soot
Poor/aggressive driving habits bad roads

Im sure I missed many other things