The PD-4104 was restyled with larger windows to create the PD-4106 which was first to use the new Detroit Diesel 8V-71 engine which developed 253 horsepower. It was also the first bus with a combined heating and cooling system with the compressor driven by the engine. Appearance is similar to the PD-4104 but the wider destination sign is an easy distinguishing mark. Greyhound and Trailways operated substantial numbers of the 3,226 produced.
Information from the Bus World Encyclopedia of Buses - 1988