Help with my truck..Please

RPM doesn't save fuel.....speed does And yet it takes FUEL to make those RPMS

 

If you lose that many mpg with that small of a speed increase then you may have another issue. The cheapest thing to check would be your inter cooler, otherwise know as a charge air cooler, or air to air radiator.
It's the radiator sandwiched between your air conditioning radiator and your main engine cooling radiator, the compressed air coming from the turbo charger goes through this before it goes into the engine intake.
If it has a big enough leak it could cause this problem.

 

Wow. I am simply dumbfounded as to how a person can "own" a truck, yet not know the first thing about it! Engine power/torque rating, transmission, rear gear ratio, tire size, wheelbase...all very basic stuff, and will all impact the truck's value and what it will be good for. Unless you've got a large fortune and are looking to turn it into a small one, buying a truck without knowing the first thing about the way it is set up is ridiculous.

My advice, sell the truck and drive for somebody else while you figure out what all the specs mean and have a greater understanding of how a truck needs to be set up to do the work you want to do with it.

 

Cummins will allow a six lbs leak in 15 seconds and still say it's good.
No leak is good.

 

Lower RPM saves fuel, just like slowing down does. When you slow down two things happen, you reduce the effort it takes for your truck to cut thru the wind, and your RPM drops. If he is hell bent on running 65 and wants better fuel mileage he needs to get the RPM down.

 

If you change tire size you need to change them all.