Looking at engine specs and transmission for fuel efficiency

If you're in the same gear and doing 60 mph at 1700 rpm, at 1800 rpm you're only doing 63.5 mph, not 65.

 

Had a 3176 hooked to a 9 speed in my first truck, loved that little motor, always ran good , not a great highway truck but was respectable running around heavy with belly dump loads

 

Series 60 14.0 deleted (proper) 600hp right under 2k torque ride steady at 70 at 78k lbs gross getting about 7.1 mpg. Don’t know my gear ratio but it’s G enough

 

Double check the trans and if it’s still an Rto not an rt you could try swapping to 11r24.5s which makes 3.90 a great a highway gear