What model vehicle do you have, and have you done anything to it already? I'd love to see some pictures...
Exactly how strict are these laws of yours? Do they cover, say, solid wheel covers? Turning a paddle-mirror into a tapered one via Coroplast or Fiberglass? Taking an exterior radio antenna and moving it inside the car? These minor mods can benefit Cd more than you think-I went from roughly 43 MPG to an average of 46 just by duct-taping my wheels, deleting my antenna (and radio since I never listened to it)
and placing some flexible tape between my body and door jamb as a seal. All for $10. If you wanted to do say, a partial Grill block for faster warm-ups, I doubt most cops would even notice!
Note that NONE of the above mods are considered 'substantial' by most people.
What do your laws say about interior mods? You may be more mechanically inclined than I am (it would in fact be impossible to be less so, heh) or you may have access to mechanics willing to tinker a bit. If so, you could play with the gearing, add weight to your flywheel, and a dozen other engine mods that other Members have already Posted here-plenty of options, and a Cop can't ticket what he can't see...under the hood.
I guess what i'm saying is-there are plenty of options to boost FE. If a massive bodymod is going to get your car impounded-just don't do it, at all. Failing that, perhaps you could fashion a Coroplast fairing, easy to add or delete in minutes with some Duct tape-then go to your local official body, show them what you want and get permission for it. If they say no, you've only wasted a little time, and if they agree then you now have an official permit to show any Cops who might try to ticket you. Even with a permit though, you'll still likely be pulled over a lot...
