50 mpg on the highway doesn't seem out of the realm of possibility.
The last time we remember three-cylinder engines being sold in the U.S. was on some Suzuki Swifts and a sister version from General Motors, the Chevrolet Metro, in the '90s. They got such astounding mileage ratings that even beaters were selling for top dollar when gas prices hit all-time highs in 2008.
"Consumers are telling us they want to buy affordable vehicles that get many more miles per gallon," said Derrick Kuzak, global product development chief. "Our new 1-liter EcoBoost engine will give consumers looking for hybrid-like fuel economy a new, more affordable choice." Read more at USA Today
(Picture right is of my dad with our 93' geo in 1993 ;-)