Auxiliary transmission for heavy haul?

I ran 2 speed rears. 5:32/3:90. 18 speed. I can’t imagine needing anything lower or higher than that for general HH.

 

My last truck had an Allison 4700RDS and 4.10 rears and it pulled 215,000 no problem even out west. Never found a hill it couldn't start rolling up from a dead stop.

Now I have a Detroit DT12 VX with 3.73 rears and so far I've done up to 150,000 with it. Same deal no problem even over the rockies.

Unless trying to get something like a maxed out 13Axle moving up a 10%+ grade I just don't see a place where I could have ever needed more reduction.

My buddy pulls a 13 Axle with the 4700RDS and 4.30s and he has no issues.

I've always run out of traction long before running out of power or gears.

I do about 99% highway and paved ground work though so maybe there's more to it for offroad applications that I just don't have the experience with.

 

I do have experience with a 22918b and a spicer 4 spd air shift aux. I loved mine and almost no issues with it. You can shift them both up and down while moving - the truck is not to also try and shift the main box at the same time. You will be stopping to re “ sync” everything.

the overdrive gear is where you will get into trouble if you try to use it under load. Bad things happen.

the fat30 cannot be shifted while in motion up or down but is super strong and allows the high torque 2050 or more versus the spicer you are not supposed to run more than 1850.

my current truck is an Allison 4700 Rds with a spicer 4 spd behind it. This combination is not ideal as you lose the shift on the fly capability for the most part. You can do it but under limited situations and definitely not while climbing a grade.

 

Thanks Ron.

 

I have an 18 with the 4sp air shift. Never run loaded in 4th (overdrive) as well.

I like it most for backing up super slow. Although we have some interesting hills in the Ohio valley that it comes in handy for.

 

But you can get no spin(limited slip). I run 3.90/5.30 two speeds. Honestly a straight 3.90 would fulfill all my needs. It can be shifted up/down whenever you want. They don’t recommend down shifting on a down grade. For the same reasons the transmission manufacturers don’t suggest it.