I drove one for a lot of years in a B-73 Mack with a 262 Cummins Diesel- - - -once you get the hang of shifting it, it's hard to imagine not having one. It's particularly useful when you need a couple of intermediate speeds when pulling a heavy load up a hill, as every main box gear becomes three speeds. Once we got the truck moving, all the shifts in both boxes were done without disengaging the clutch.
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!