N14 with 3.91 gears, 10 speed. Change to what gear?

You can up that to 430hp and get 3.55-3.58, you will be absolutely fine. 1500-1600 RPM at 70mph is good, below 1400 is not.

 

That is gonna be way more expensive than getting new rear ends. A reman 13 speed is $4000-4500, then you need a $3000 core, cut driveshaft, get new shifter and some other small parts.

 

The key is letting the engine “breathe” let it work in a spot were it is not being force fed air and heat. Low manifold pressure is best. Running with the turbo spooled up all day just kills engines. I can cruise 1900 rpm at 65 all day and barely make 10 psi manifold 50 on fuel pressure and 600 degrees on pyro and get job done just fine. Rto 13 speed 4:11 rears. Still have plenty of throttle to get outa the way if needed.

 

I wouldn’t even waste money on a reman, it’s like $800 more to buy a brand new one with no core
tower should be the same, it was under $1000 excluding the trans to swap the last one we did and we paid someone to do it

 

Oh wow, where did you see a new one for $5k? All I saw $8k and up for new

 

$800 more then reman+ core

 

What CPL is the N14? What size tires are you running? The N14 isn’t one to pull down below 1500 RPM, they need to stay in that 1500-1900 RPM range to live a long, happy life.

 

IMO if you're replacing the rear end strictly for fuel economy you're wasting your time. 7.3 mpg is very good fuel economy IMO. I doubt the gains you'll see will offset the cost for a very long time.

Unless you've already got the carrier assembly laying around or you've got a lot of miles on that diff I would just run it as is until its time to replace.

 

If you’re really getting that mileage I wouldn’t touch a thing. Those Rpm won’t hurt it and as others have mentioned low rpm wears the main bearing caps making the block junk or very expensive to fix at overhaul time

 

Leave it be.That fuel mileage is very good especially on a non aero truck.Keeps the rpm’s up and doesn’t lug the engine.