The General Motors PD-4104 was a 38 or 45 passenger Parlor series coach built from 1953 to 1960 in the United States. It incorporated several of the PD-4501 features such as large forward slanted picture windows, air suspension, air conditioning and an optional restroom. 5,065 were built in total. What made the 4104 so special, and how did it come to utterly dominate the market? Several factors, but first and foremost, it was just plainly superior to any other bus being offered at the time. Let’s look at the details.Weight. Rather than use a separate body and chassis, 4104’s were fully monocoque-bodied (similar to the Gar Wood coaches we looked at in our Aerocoach post). High strength aluminum panels over the monocoque frame made for an extremely light yet rigid coach, easily able to stand-up to the constant over-the-road pounding an intercity bus endures. Power and mileage. The relatively light weight 2-stroke 210 hp Detroit Diesel 6-71 engine, which was then an exclusive to GM coaches, combined with the light weight of the coach, made the 4104 North America’s most fuel efficient bus, and most likely one of its faster ones. A 4104 could typically get 8-10 miles per gallon of cheap diesel fuel compared to 2-3 mpg for the ACF Brill using the Hall-Scott gas engine, or 7-8 mpg for the Fitzjohn and Aerocoach which used the four stroke Cummins diesel. And it had a cruising/maximum speed of 65 mph.The long gear shift lever was some 30 feet away from the transmission, so it required a deft hand. Upshifts were relatively easy, and the better drivers managed it without a clutch. Downshifts required double clutching and revving up the engine to the required speed in order to match the rotation speed of the gears on the output shaft. Pulling off a downshift without any hint of gear grinding was the sign of a superior driver. the 4104 was a revolutionary design when launched – more modern looking than any other bus (or car) on the road. It created as much buzz as the ’63 Stingray or ‘ 66 Toronado. The full aluminum-encapsulated body and forward canted large windows became iconic design elements on buses for the next 30 years or more. Everyone rushed to imitate the 4104.