I've done it again! That's three times in a row; I don't think this can be considered a fluke. 54.16 MPG in my Civic Hatchback.
I'm going to run one more tank before I start making modifications. A tune-up, an air filter with better flow, and synthetic oil are definitely on the table. A fella I respect has been experimenting with his Corvette (!) and swears he got a 10% boost from a product called "SweetShooter". Originally intended to coat gun barrels to keep them from getting fouled by black powder, it has been used in oil and gas to coat the cylinders.
Heck, if he can risk his Corvette, I can I can offer my HB. it's a dirt-simple mod, and I can even have it ready before the next tank. 60 MPG, here I come!