Or maybe a train horn...
The mind boggles.
Thought of this one over pizza tonight (hey, I can do the dishes and laundry now that the sun has gone down!):
Horse hoofbeats, timed to your speed. Slow gait in stop-and-go traffic, maybe with some frustrated snorts as the car sits still in traffic (it's so damn gratifying to sit still in an electric. Unless you're running the AC).
Full gallop as you're tooling down the highway!
Of course, any number of theme songs for speedy shows and movies would work.
The mundane folk would simply have a setup that imitated a normal car engine noise.