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

I have a 96 Freightliner Fld120. It has a 10 speed with 3.91 gears. It's a single semi with a GVWR of 54,999. I haul oilfield equipment mostly. Sometimes I go up to the mountains but usually stay in Texas, Louisiana Oklahoma. I'm around 31.5k empty weight so only haul 23k or less. My current rpm at speed are these:
60mph 1500 rpm
65mph 1600 rpm
70mph 1750 rpm
75mph 1850 rpm
I'm averaging 7.3 mpg but wondering what gear would be better to get higher fuel mileage and less wear on my eengine. I am usually around 70-75 range.

Any info appreciated.

 

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 forgot to mention, this is a 370hp n14. I'm guessing since it doesn't have a lot of power, it will waste more fuel and more wear & tear to get it in the 70-75 mph range. What is the ideal rpm I need to be in when in those speeds?

 

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