Fuel mileage question

If you went from 3.08 down to 2.64 you are forcing the truck to give up pulling against it's load for a taller gear. You will cruise 70 on less RPM, but also you will penalize your entire truck by seeing it fall down and be unable to run the smaller hills and then downshift more to overcome them when heavy.

3.08 aint half bad. Your average 6.4 aint half bad. Hold on to that. It might have been better to have the double under instead of over but eh... there are a thousand ways to build a truck.

Where you run also matters. If it's steep enough you will discover gallons per hour (Around 30 or so all out) to pull it.

 

How many miles? I was driving a brand new Columbia, and it got similar mileage until about 60,000, then it started getting a lot better mileage.

 

I kind of doubt you will see enough gains to make it worthwhile paying for the rear end swap and you may not have the transmission/engine to make it pull great. I am not familiar with your set up. Eaton has a great road speed calculator to help with specking a truck for the speeds you plan to run.
I built a glider in 2015 with a 500 hp pre EGR series 60, 18 speed, 2:64. I also have a ported and coated manifold, slightly larger turbo, FASS fuel system, fleet air filters and a Pittsburgh Power Box which I run on power setting 3 when loaded. The box produces 75 additional hp at this setting. I geared it to run in 16, which is direct drive and the most efficient gear position at speeds up to 65 mph and 1550ish. Single over drive in 17 and good to 75 or so at 1500 ish. I rarely use my double overdrive 18, but it is there when I need it. I can’t speak to how your transmission and engine hp would pull hills with a 2:64, but mine pulls like a champ. I hit the hill at the correct rpms and I have the horsepower to effectively pull at 80,000 and horrendous aerodynamics. When I was mty 50% of the time I got consistently up to 6 mpg from 5, which is great for a car carrier running 65-70.

 

Wow that seems very low mpg to me. Not sure on my gearing but (1550 rpm is 65mph) with low pro 24.5 and a 10 speed with a Detroit 60 series I’m pulling 6.5mpg from Memphis to Philadelphia loaded and I break 7.5 empty. Even break 8 empty in Texas. This is with a W900 to boot.

 

It sounds like the tire break-in period, which is 35k-50k, according to the Cummins MPG Guide/Study. It’s also possible the truck/engine itself has a break-in period.

 

The salesman said we would get 8.5-9 mpg.

Fixed it fer ya.....

 

I think you're doing good for 100k pounds and running 68-70...

Try 62...see what happens