General Motors is investing $4 billion in its U.S. plants over the next two years to boost the manufacturing of gas and electric vehicles.With the multibillion-dollar investment, the Michigan-based automaker will be able to assemble more than 2 million vehicles per year in the U.S. The latest investment comes just two weeks after the company announced it earmarked$888 million for its Tonawanda Propulsion plant near Buffalo, New York, to support production of its next-generation V-8 engine.Prior to the inve ...