I had some communication with a customer support person in Georgia who represented that he had formerly worked in the powertrain engineering department. He told me that Blue Bird had never done the engineering work to support the 6-speed option in a rig like mine. I'm not sure just how alike it would have needed to be -- I assumed at minimum something with the same axle ratio (mine is 5.3
, probably MD3060 or B400 tranny (mine is B400), and 8.3L Cummins (not sure whether it makes any difference whether it's the C or ISC model). Generalizing a little more, and based on the observation that there aren't very many unique powertrain combinations available, I infer/assume that there probably don't exist any Blue Birds with 6th gear enabled from the factory.