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

Stepping up to an 11 or 12 22.5 will probably give you just enough to be happy with

 

I have two trucks I've converted from 22.5's to 24.5's to get the RPM's down on them. It's been tough getting my drivers to wail on my big cam and N14's when they're used to my older 3406 Cat powered trucks. 1900 RPM on that N14 is just fine and your mileage isn't going to get much better going to a higher geared rear end.

 

You’re doing good if you can even get a driver to sit in a truck that old…I have pretty much given up but refuse to buy new stuff because of what I see as unnecessary complications so I guess I’ll just enjoy my classics by myself