By David Shepardson and Joseph White TRAVERSE CITY, Mich. (Reuters) - The complex U.S. emissions rules driving the auto industry's technology investment should be overhauled, auto executives and environmental regulators agreed Tuesday at a conference here, even as they differed sharply over how. Chris Grundler, head of the U.S. Environmental Protection Agency's Office of Transportation and Air Quality, said regulators "should be open to a new policy framework post 2025" to drive down greenhouse gas emissions from cars and light trucks.