The 2-cylinder TwinAir 875cc should be the most fuel-efficient gasoline engine in the world, thanks to regulation-busting 57.4 mpg fuel economy. From "Autopia":
"The parallel twin uses four valves per cylinder. Electro-hydraulic intake valves with variable timing in place of a conventional camshaft allow more precise combustion, increasing fuel economy and torque. Fitted with a tiny turbocharger, the TwinAir produces 85 horsepower and 106 lb-ft of torque. An 'eco' mode cuts that to 73 lb-ft to boost fuel economy further."But it's not just about fuel-efficiency numbers -- reports are that the engine can still boogie. It will do zero to 60 in 11 seconds and tops out at more than 100 mph (107, to be exact).

OK, OK -- 11 seconds is a bit much. But remember the part about 57 mpg?