Won't happen. At most it will be 2-3 tenths, I've done the testing, speed makes far more difference. Don't believe the salesman's hype "spec it right and you can run fast with fuel mileage".

 

You can gear a truck to run at 70 mph at 1200 rpm but what folks leave out of the equation is how much more boost you'll be using to turn the wheels. It takes fuel to raise boost. You'll be pushing a lot of air at 70.
You want your truck to cruise along around 5 psi boost on the level at most for best efficiency. How many times a day do you stomp that boost pressure over 20? It's just dollar bills flying out your stacks.

The key to more mpg is slow down. You can't change the laws of physics any more than Obama can control the Earth's climate. Although there are many fools who think otherwise.

The less air you push the less power (fuel) you use.

Gears provide leverage not power.

 

Why is everybody talking fuel mileage? The OP needs to change for startability and gradeability.

 

I just did my axle ratio change. Total downtime - 1 day. Brought the truck to the mechanic in the morning and it was ready at 10 pm. Paid Cnd$2,100 to the gear guy and $650 to a shop that did uninstall/install. I told them to use the same oil as I just replaced it a couple of months ago.

My old ratio was 3.73 and with the low profile tires I was doing almost 1,400 RPM at 60 MPH. With the new 3.42 ratio, the engine spins 1330-1340 RPM at 62 MPH and when I'm fully loaded I stay close to 1,350-1,360 RPM or 63-64 MPH. Most 2-lane highways here in Ontario, Canada have 50 MPH speed limit, so I just use 8 Low in my 13 when I have to use these highways.

Gradability has naturally decreased but at least now I can maintain 62 MPH without killing my fuel mileage. With 3.73s, the engine did 1440 RPM at 62 MPH so I never even ran at 62.