I don't see any purpose in saluting when actually driving (save going through the gate where it is legit). Depending on ambient conditions (darkness, rain, sun glare) it is likely they won't even see you return the salute, AND if anything it could possibly be dangerous given the right circumstances. Of course I don't wear my cover when driving after going through the gate, so it would be a moot point anyways.
When I was in ROTC, during the first week of my freshman year I rolled by the CO/XO while covered in my car, and saluted them. XO just looked at me like I was completely retarded, and I have never saluted in motion in any vehicle since.