I am in the process of buying a '13 Dually aisin, and tried out two duallys first, one with 3. 42 and one with 3. 73. I hitched them both up to the same 10k trailer and pulled the same 6% grade. One direction I just put it in tow haul and let the transmission do its thing, the other direction I downshifted it myself at the bottom of the hill.
3. 42 had a hell of a time holding 6th while getting to the grade, 1-2% climb getting to the steep part, once there it dropped to 4th, went to 5th, dropped to 4th, went to 5th, all in about 1. 5 miles. Down the backside it gained 10 miles per hour and the "smart brake never did engage". Back up I manually put it in 4th and it held fine, couldn't gain any speed though, hit it at 72 and stayed there. Switched brake to just on position and it held great and would lose speed in 4th, worked great in 5th
3. 73 would hold 6th, at the grade went to 5th and stayed. Smart brake allowed about a 5mph gain, no more. Back up the other side manually in 5th and of course held speed, manually to 4th and I could gain speed, forgot to try the brake in "on" mode since the smart brake worked so well.
O'le Mr. Barlow will probably remember my last truck and the 3. 42 towing saga, I too felt that if the final drive was the same by downshifting the transmission, however I'm living proof that it didn't work worth a hill of beans. I had to manually shift the transmission and manually use the brake every time. If I let the truck do its thing it would drive me nuts.
Now I will never admit Harvey was right
but I had my suspicions on the new trucks so that was why I tried it.
Although the aisin is a damn good transmission, it still doesn't work that well with a 3. 42.
I'm picking mine up with 3. 73 tomorrow, will report how it tows 24,000lbs soon.
Attention: TDR Forum Junkies 
:-laf
