Aero improvements?
 

What can you do to these cars to make them more aerodynamic? I'm trying to increase gas mileage.

Spending a few hundred (or a few thousand) dollars to save a mile per gallon isn't the most prudent way to spend money on "upgrades". Really the only big aerodynamic improvement would be headlight covers.

Where can I find these headlight covers? And I have a feeling right now my car is quite a bit worse aerodynamically than it was before because it's missing some stuff off the front end. The guy I got it from said he tore some sort of skidplate off and threw it away because it was dragging and I know those can make a decent aero difference on some cars. I don't know what else is missing but I'm sure there's more.

Clear covers: Clear Headlight Covers, 79-83 280ZX

Tinted covers: Tinted Headlight Covers, 79-83 280ZX

Body / aero kits: 79-83 280ZX Body Kits/Spoilers/Wings