9MPG, is that possible?

Easier to have the factory do it than it is to pay someone of their own to do it. When you buy in bulk though, the extra expense is nullified usually.

 

Swift used to spec their FLD Freightliner trucks with direct drive 10 speeds and 3.36:1 rear ends. Which made them worthless at the end of their service with Swift. So when they were put on sale, SelecTrucks offered to swap in a 13 speed with a .86 overdrive gear.

 

Last year I did a trip from the Petro in Laramie, WY to the flying J in Gretna, NE. I was hauling rock salt from Utah, and my truck weighed about 76K gross weight. I kid you not, I managed to get 9.4 mpg for the day. I even kept the cruise control on 65 for the majority of the trip. So, depending on how gently you drive, yes it's very possible.

 

My friend's 2015 prostar with cummins isx15, 10 sp ultrashfit, 2.64 rear axles, and loaded at 19k lbs. Gets 9 mpg when driving 70 mph.

 

We're lucky to get 6 the way we drive haha. Of course we drive to deliver, not for fuel mileage.

When I was otr I could take my time getting up to speed and drive for fuel.

 

My t600 best mpg is 5.7 mpg. Still have a stock mbn engine.

 

I get 9mpg OFTEN!! As evidenced by hand calculations and over 8.5mpg monthly and quarterly bonuses.... In fact averaging 9.4 hauling 24k right now in Tennessee.