It's been done, not worth the effort except for bragging rights. The small things do add up but the biggest way to increase mileage is to change driving style and lighten the car. Much easier than putting a huge body kit on the car and cost effective.
He put a large wing on the beetle to improve aero. There's more drag is on the trailing edge, which is also confirmed on his site. This is why solar racers or other similar ultra low drag cars are shaped like teardrops. This is not what F1 racecars or drag racers look like because they also need downforce and traction.
If you search for pictures of the honda insight, the rear wheel wells are covered. This reduces drag. The front wheels need clearance to turn so this is not practical.
The wheels could be closed discs but this needs to be balanced against brake cooling. I feel a good compromise is a flat wheel with scoops to aid brake cooling. Something like this but flatter
The underbody of the car could also be flattened with panels.
Like Fluid said, this all costs money and time that you will not recoup in savings. Changing driving style is as easy as changing your foot position. You could lighten the car by removing the interior trim and spare tire. If you wanted to get crazy, replace the hood and trunk with carbon fiber parts and replace the windows with lexan plastic. Of course, this is way beyond any resonable cost-benefit ratio.
You could take some cheap foam and fill the front grille but this is the only significant non-noticeable cheap body mod that I can think of.
Personally, I would rather eat any losses in mpg rather than tear out the trim, put body kits on the car, etc. It's a fun excercise but IMO not worth it.
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