General Motors Corp., the largest U.S. automaker, unveiled a production version of its Volt electric car, featuring better aerodynamics that will improve fuel efficiency by up to seven miles per gallon.

The Chevrolet Volt, which is designed to drive 40 miles solely on an electric charge, has more rounded features than the preliminary version shown to the press and public, Bob Boniface, the model’s design chief, said in a presentation in Traverse City, Michigan.

Boniface said Detroit-based GM still plans to begin selling the vehicle in 2010. The Volt will be powered by lithium-ion batteries that can be charged with an outlet. After the battery charge runs out, a gasoline engine turns on and recharges the battery.