Aerodynamic modifications can significantly improve the fuel economy of Geo Metros:
metrompg.com
I've always been fond of taking the gas engine out of one and slipping a 10-15 horsepower diesel tractor engine in one. 60-65 mph top speed and instant 80+ mpg while able to be run on waste fryer oil, although acceleration will be painfully slow. With aero mods and LRR tires, could jump into the 100+ mpg region, with a slight top speed increase.
Need more power? A VW Golf TDi is what you need. That would make for a 60+ mpg evil Metro with 0-60 mph < 7 seconds.
The unnecessary felling of a tree, perhaps the old growth of centuries, seems to me a crime little short of murder. ~Thomas Jefferson