This squat four-seat hybrid sedan previews a new family of plug-in electric drive systems that G.M. calls E-Flex. The system, which the company plans to begin installing globally when the battery technology is mature, will be capable of delivering the equivalent of 100 miles a gallon or more in urban driving, G.M. officials said. The Volt’s total range is 640 miles using the combined capacity of fully charged batteries and a built-in gasoline-powered generator.