W900 that's way too slow.

65 mph was my average. This was pulling flatbed and wheelbase was 265.

 

I have 3.36 and I was getting north of 6 until winter fuel.

 

With a 6NZ or a 12.7?

 

The point I am driving at is assuming a jump in 1 mpg and really getting it is alot of different. The op said he wants to run 70. Now with 3.36 they are high enough to run in direct at 55 and .87 at 65. So less overdrive less power loss to heat.

I would also be willing to bet the OP has a minor engine performance issue causing bad mileage.


Again there is just no way of knowing how this change will actually affect mpg. Numbers on paper look good. Till the real numbers hurt your feelings after dropping a wad of cash.


If road speed is all he is after by all he is after 3.55's will get him there

 

Mines got 3.55’s on lp24.5 and a 10 over

It’ll get me a high speed award

 

12.7

 

It's hard to find BSFC numbers nowadays, but if you look at these numbers for an C-15 industrial engine, you'll see that the BSFC only varies ~10% from highest to lowest, and far less than that generally.

That makes the idea of an rpm change resulting in a 20% mpg most likely impossible.

 

If someone has put a jacked up file in the ecm it can hurt the fuel mileage. The op is also comparing two different ecm’s. If the 40 pin has a pre-low nox it most likely will get better mileage than a 70 pin stock low nox.

 

Throwing tall 24.5s on it is probably the cheapest way to see if gearing change will help. I'm around 5mpg with 3.55s pulling my hopper through a lot KS, OK, NE, SD, occasionally into MO, AR, and IA. I'm always running 5 or so over the limit and anywhere from 80k to 92k pounds. Another issue is when you go from those 4.11s to 3.55s you may suddenly be dropping gears on hills you weren't before.