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

For your application 3.42 or 3.55

 

3.58 would be easy to find.

 

3:55 - 3:70's would be fine. The 10 speed limits you somewhat - I'd just drive a bit slower and leave it how it is, unless time/loads dictate a need to change/ or they're up for a rebuild. The N-14 has a lot of pull with 15L displacement.

 

I wouldn't complain about your MPG and know that if you spend the money to swap gears it will likely take a long time to see return on your investment. Granted you only have one pumpkin to swap so it's considerably cheaper. I guess you could swap it out and call it preventative maintenence.
I just wouldn't expect a huge gain

 

Rear gear is Perfect set up. 7.3 is good. Rpms are right where they should be. Leave it alone. Mileage will probably drop down in the 6’s because You will be working that engine harder to do the same job. If anything a Rto13 speed would be a cheaper upgrade

 

If you have any 55-60 mph roads I’d swap in a rtlo 13 so you can get the rpm’s back up to 17-1900 where they belong unless you like pulling engines for fun to machine the fretted mains

 

I think RPM is irrelevant, it can run 2000 rpm all day and not hurt a thing.
For best MPG watch your boost gauge not RPM.
If I'm trying for good MPG I'm better off at higher rpm and lower boost vs low rpm lugging and higher boost pressure

 

16-1900, fuel cut off should be 2350

you’ll cause fretting on the mains pulling much below 1500, n14 is not a motor you want to lug