The chairman and chief executive officer of Indianapolis-based Allison Transmission Holdings Inc. has announced plans to retire. The company says its board of directors has entered into a new employment agreement with Lawrence Dewey which runs through May 31, 2018, at which point Dewey will retire. The Allison Board of Directors says it will implement a transition plan ahead of Dewey’s retirement and will consider both internal and external candidates to succeed Dewey as CEO. Dewey says he plans to work closely with the board to ensure a smooth transition. Dewey is the longest-serving CEO for Allison, having served in the role since 2007. He led the company through its transition from being a division of General Motors Co. to its current status as an independent public company.