The horsepower size of the engines that are being used by the highway hauler trucking industry has been steadily increasing. The most obvious advantage for using a gas turbine engine for this application is the significant savings in weight and size over other types of prime movers. The mechanical transmission, for use in the heavy duty highway hauler tractor application, has been the standard of the industry because of its advantages in reliability, cost, efficiency, weight, size, and ease of maintenance. This paper describes some of the developments and improvements which have been made to the mechanical transmission that practically eliminates all the skills and effort required to operate the new generation of gas turbine highway hauler tractors. The purpose of this paper is to present some of the approaches which have been taken to optimize the mechanical transmission for the heavy duty gas turbine tractor application by proper selection of gear ratios to improve vehicle performance, and by the utilization of newly developed controls and drivetrain components that produces a more versatile mechanical transmission.