Hmmm...best MPG car I ever owned was a 1979 Dodge Colt Hatchback purchased for $500 back in 1995 at Nashville TN. It had a 1st Gen Mitsubishi 4cyl with RWD, built on a sturdy 1700 lbs frame, and was more of a micro Station Wagon than you would expect-clutch and 4 speed manual, lots of room everywhere, went up and down Tennessee hills and tight curves in the rain like a dang sportscar and got between 45 and 50 MPG...how? NOT A CLUE.
What's even weirder? This vehicle is listed in the online specs as having FWD-but I know for a FACT that mine was RWD because I wrenched it myself-did I get some weird experimental model? Who knows? At any rate I had to go back to NC for Emergency Hernia surgery and sold it to a co-worker for $1000...three months later the IDIOT locked up the motor because he forgot where the Dipstick was and never checked the oil levels...
That Colt deserved better.
Are there any chances of a Chevy Sprint in your area? A precursor to the Metro LS, the Sprint was a 3-cyl FWD 5 speed manual like the Metro, but didn't have the A-pillar rust-out issues of the Geo and was even lighter! A friend of mine in Fremont NC had one, flogged it like he stole it and reported high 40's in the MPG dept. and almost no maintenance...if he hadn't sold it to a parts dealer after some idiot caved in his passenger door i'd have made him an offer and grabbed a hammer...
Had a '68 Beetle and a '72 Superbeetle during my life-never got over 38 MPG at highway speeds...but if you find a used or wrecked one dirt cheap and want to reduce the horrible Aero with a Roadster kit or even roll your own chassis with foam and fiberglass-well, who knows what might happen?