Mileage Question

21 MPG is pretty good.
Gears in rear end will have an effect, as said.

I would look at maximizing each aspect you can:
Tire inflation
Fluid changes (switch to synthetic, or at least fresh, full new fluid),
Adjust drum brakes in rear (possible car has front drums to) to point where brakes are BARELY touching drums,
electric fan,
Air intake tube, for more air flow to air cleaner (warm air is better for MPG, so draw from behind radiator)

I personally wouldn't go to extreme on body modifications, but the one I would consider would be a belly pan from under the front bumper, to the front K member. There is a lot of the K member showing that acts like an air plow. A sheet of plastic from sign store (think the material that they make election signs with) would help direct air that is coming under the bumper smoothly under the K member, instead of the K member catching the air.

Hypermiling techniques from www.ecomodder.com would help. As previously said, some are extreme, but changes to driving habits, and adjustments for efficiency on the car, can go a LONG way.