Most of the 2-speed auxiliary transmissions are underdrive/direct ratio. They allow an extra gear reduction to each main transmission shift, effectively splitting each gear before making the next upshift. It's a good way of keeping a heavy load moving without dropping the engine RPM out of the effective power band between shifts. That type of transmission doesn't do anything for top road speed- - - -it just keeps the engine it its optimum power band more of the time. A 3 speed auxiliary also includes an "overdrive" ratio for each main transmission gear. Older Diesels, especially Detroits, needed all the gears they could get, since the usable power band was only 400 RPM, like 1750 to 2150.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!