[Christ]

If that's the case, maybe going diesel isnt' the most viable option, since any Diesel option you choose will ultimately leave the vehicle in a state of down time until it is completed and inspected for legality.

That said, consider starting with aero-mods and operation-mods, and use the money you save from those things to get a more aerodynamic chassis to start with, so you can shoot for a lower-powered (read: more efficient) diesel engine to swap into it, with less federal regulatory
"I'll stick my nose in your biz whether you like it or not because something doesn't meet an arbitrary figure controlled by an overpaid beer-o-crat who says you're now in violation of some outdated law or rule that was only created to inconvenience people until they realized that we were right even though we were actually wrong because we have federal backing and can make YOU believe whatever we want by introducing those arbitrary numbers to your schools and workplaces and forcing you to cooperate with our demands under penalty of prison and fines even though we've never been a REAL government agency." agency interference and finger pointing in your general direction.

Yes, they will try to interfere, even if you're doing something that follows their protocols, because it doesn't officially qualify as "eco-minded" or "environmentally friendly" unless they say it does. They're NOT wrong. Evar.

If you don't believe me, ask the 50,000 species that went extinct in the time it took me to type this post.