Removing the belt driven fan and putting in an E-fan frees up horse power AND gets better MPG, one of the few changes that helps both.
An OBD II live scanner will show you your fuel trims, there are Bluetooth units that work with most Smartphones, under $40
This will help show if your engine is running correctly, so best MPG
Good spark plugs and running a can of Seafoam in the gas tank once a year will help keep performance and MPG up.
Rangers have the aerodynamics of a brick, and no, running with tail gate down doesn't help.
Thinner tires help with MPG, less friction, but also lower traction.
Depending on your use changing the rear end gearing to match your use will help longterm.
Look here to see what gearing you have now:
http://www.therangerstation.com/tech_library/axle_codes.shtml
4.10 gear is best for pulling and hauling, but RPMs are high at highway speeds so MPG would be lower than other gearing.
3.45 gear would have lower RPMs at highway speeds so best MPG but wouldn't be good for hauling, and slower acceleration from stop.
The sad news is that gasoline engines are only about 25% efficient in converting gasoline into power that turns the wheels.
75% goes to heating up the air around the vehicle

So from a $4 gallon of gas $1 dollar spins the wheels, $3 heats the air.
Diesels can get near 40% efficiency