It's not the single most fuel-efficient subcompact on the market--that'd be the Toyota Prius C hybrid--but the 2014 Chevrolet Sonic will deliver real-world gas mileage of 30 to 35 mpg when fitted with the optional 1.4-liter turbocharged engine. It's optional (on LT and LTZ models) and rated at 31 mpg combined with the automatic transmission, rising to 33 mpg with the six-speed manual.
If you drive sensibly, you can most likely do better than that--we got 34.2 mpg over several hundred miles in a 1.4 sedan with the six-speed manual gearbox. And the turbo--unlike many earlier such engines--doesn't require premium fuel, but is quite happy on regular unleaded gasoline.
The base 1.8-liter engine costs less, but it drinks more fuel. Its ratings are a less respectable 30 mpg combined with the manual, falling to 28 mpg with the automatic. It's also less fun to drive, so we think the payback is pretty clear.