Here's what you gotta look at whenever it comes to mileage increases, which are usually minimal.....
How much would you spend on product A, B, C and D?
How much fuel would that product buy?
How much of an improvement did you really get?
How long would it REALLY take to pay off?
Whenever you start talking gear swaps for mileage, it doesn't work out well because you have to take into account.....
1 - The price of a set of gears (per axle)
2 - The price of a seal or rebuild kit (per axle)
3 - The price of the labor per axle
4 - The price of the fluids
Typically, a gear swap on a 4WD truck goes like this, PER AXLE
Gear set: $200
Install Kit: $60 - $180
Labor: $300 - $500 per axle
Fluids: $24 - $72
Total per axle: $584 - $952
Here's a hypothetical example.....
Let's say you pay the mid point price PER AXLE, so for a 4WD you just paid $1536.
Let's say fuel costs $2.65 per gallon. That means you could have bought 579 gallons of fuel. At 30 gals of fuel, that's 19.3 tanks of fuel.
So, let's say the you gain 1 MPG by doing this swap and you're now getting 15 MPG.
In order for that gear swap to pay for itself, with a 1 MPG improvement, an original cost per mile of $0.189, a new cost per mile of $0.176....saving you 1.3 cents per mile. You'd have to drive 118,153 miles to break even.
Best bet is properly inflating the tires, keeping an air filter clean, using the lightest weigh oil possible, removing all the excess BS that isn't needed in that monster tool box that just stores junk, keeping the RPM's under 2000, boost in the single digits and EGT's in the 550 - 700* range.