The main reason I purchased the Geo Metro was as an MPG experiment. I had the crazy idea I wanted to build a 70MPG car. The Metro was in horrible shape when purchased and I got less than 30 MPG on the drive home. That would have been fine for a normal car but the EPA estimate for a '93 convertible Metro is 37.7 MPG (20% higher than what I was getting). After rebuilding the engine I started slowly working on the MPG aspect of the car.
My MPG graph from ecomodder.com:
Improvements/modifications I have done:
Increased tire pressure from 32 PSI to 50 PSI
Swapped in a transmission from a 1990 XFI
Installed a 3-Tech economy grind camshaft
Installed a 3-Tech 10 degree advanced camshaft sprocket
Full radiator grill block
Installed an MPGuino to monitor instantaneous MPG
Removed passenger side mirror
Note - The car was sold in early 2011 so the project was stopped at the time. Plans are to start another MPG project at some point but it may take a while to get going again.