That explains it, I think the fifth forward gear was a fairly recent development, at least it was the case with Nissan. I remember we had an old van with only four, then we got a newer model, it had five.
Lots of vehicles here have 5 speeds. The Toyota Celica I had was a 5-speed. Even a lot of automatics have five speeds. With the miles of empty highways in this country, a 5-speed engine is about the only way you make get some places quickly without using up a lot of gas, or putting lots of wear on the engine.