old 13 double over

I assume you are wanting to go faster with lower rpm. I would get the rpm reduction from the rears. 3.36 or 3.55. 13 speed and you will have all that today's speed limits and fuel cost/freight rates will allow. Free advice and worth every penny.

 

I saw a front diff running 250F in conditions like that. Changed to 75W-90 synthetic and the temp dropped to 175F. Don't thank the 250F hurts anything but the ride is more relaxed at 175F.

 

IMO you also want to keep an eye on the power curve of the engine. Dropping the R's doesn't guarantee lower mpg's. I found the power curve on my dog was tight and ended up with a 13 UD. Most curves dive past 1800

Being said ..I was trying to stretch out a 380 hp to pull 44k and get 6 mpg. It wasn't in the cards lol. Engine just had to work too hard pulling 44k. 505 Yamaha would have been a much better power plant.

There is no substitute for cubes!

 

I just got a w900l with a Detroit, 13 speed and 3.70 rears. I wanted to change the rearends to 3.36 or so, but I am at 1,600 at 63, and that is the sweetspot in the 12.7. if I put it down to 1,300, I would be lugging it, and mileage would suffer.

 

There's some owners, that take your setup. Run around 2.80 of so on the rearends. And if there 13 is a OD. This allows them to run in direct or 11th gear at around 62 MPH. But when in a hurry or empty click it up to 13th and run out at a faster speed. Also the 12.7's I ran from the time they came out. They loved pulling at lower R's. Shoot most will dig and pull like crazy at 1150 or so. People that cried about them not pulling. Was not giving the engine the change to put it down to the ground. I got to drive my first 12.7 in a 1989 Freightliner. And the 11.1 before the 12.7 and it did a great job for a 11 litter engine.
My next truck will if I can find it. 12.7 DDEC IV and 13 speed and 2.70's or around that rearends.

 

There's a plaque on the dash that says DDEC I I,
Minn hard pull 1,400
Best economy 1,600
Operating range 1,600-1,800
Max boost 30 lbs

I've found with this one, and my DDEC III, that neither liked pulling at under 1,400 rpm.

 

What everyone looses sight of is empty or not the force of the wind pulling that tall a gear is a loosing battle for fuel economy and wear and tear. I ran a .62 overdrive with 3.70's for 15 years. After running on 3.36's and a .79 overdrive I will never go back to that whining needing a back box every year pos. Not saying my combination is for everyone or the greatest but it works for me.


Buy the truck you want and need not one close hoping to change it over. Change is high plain and simple

 

Bout 150