gear change
 

I'm running an old FLD120 with a 400 N14, 10 direct, and 2.64 gears. It's just about due for an in-frame, with around a million miles.

If my math is right, a 13 speed with .78 OD and 3.36 gears would give me essentially, the same final drive ratio I have now. The same 13 speed with 3.23s would lower my cruise RPM from about 1500 to 1400 at 70 mph.

Would I see any appreciable gain in fuel mileage by swapping the transmission and gears? I'm getting about 5.25 right now, running 62-68 MPH, gross loaded weight 90K with a tri-axle trailer.