I have a Spicer 5831C in my '46 two ton. It splits the crash box under and over in each gear of the main box, and can be shifted progressively through all 12 combinations if desired. Although the first 6 combinations in my case will get you up to about 15 mph with the engine wound up.
Grigg is correct in that the OD is best used in the higher gears of the main transmission to limit the strain on the OD gear set. Its not that the overdrive is made of glass though. As long as the operator is using a portion of his head and has a foot made of something other than lead, there are situations where overdrive can be used with the lower main gears.
I must admit, 1st over is a nice combination for idling through some parades....
I find that the Spicer isn't that difficult to shift, and it quickly becomes second nature to slip it into OD as the last shift getting up on the highway, and its the first gear to come out of on a downshift. Even loaded I find that splitting anything below 3rd in the main transmission doesn't gain much for the effort.
