15 speed direct drive with 2.85 rears slow up hills and poor mileage

What do you call a hill? Figure out what your final drive is in 6th.

You can always sell the rears to Richter.

 

What's your Torque spec on it?
I agree with putting it on a dump and checking the power output. A pittsburg power box might do wonders for this truck.

 

A 15 where you split the top end?? Every 15 I've ever seen has the deep reduction gears & either an overdrive or direct.

 

If you split the top end its not a 15 it's a 13spd.

 

Exactly, I don't mean to be rude & don't claim to know everything about trucks. But, some of the questions people ask on here make me shake my head. Its a wonder most of them can tie there shoes let alone move a truck down the road.

 

24-26psi is a bit low for a single turbo,way low if it's a acert. Have you checked the air to air for leaks? Last cat I ran was a mbn,it was in the 32-34psi range.

 

I'm not ruling out motor issues. With that said that trans is not set-up to drive those rears. A 15spd is a glorified 10spd. Great trans durability wise and nothing wrong with 15. But to drive 2.85 rears loaded heavy with any kinda conviction you need to be able to split gears on a pull. Otherwise you'll be doing a lot of straight sticking. In my opinon the 15 is holding the truck back. Had a bunch of 15s over the years and on everyone that I changed to a 13 the trucks got better mpg, better top end, better pulling power on highway. Off-road the 15 has deep reduction for better low end hands down.

 

My friend just did that and you are 100% right on the money, a whole different truck with the 13.