Which transmission?

Engine RPM divided by transmission ratio (high gear: .73 for B-ratio Fullers/.87 for A-ratio fullers) divided by rear end ratio times tire diameter (measure them: 275/80x22.5 about 40":11x22.5 about 42": 11x24.5 about 43" times .002975 (conversion factor) EQUALS road speed in MPH

SO: for my little eagle:

1500 rpm/.73/3.73x40x.002975=65 mph

Or use eatons road speed calculator.
Play with ratios. Gear it for top hole at the speed you want to run on the highway.
At 100k, coming out of the hole, I'd love an 18. Otherwise a 13 is fine for most hills. 10s are OK for flat and light.

 

For New England I have 18 speed 3.73 rears and tall 24.5 rubber. This combo does alright ..... But your not going to get those big mpgs #s you see posted around here....

 

And driver preference or style of driving. I agree.

 

No a RTLO 18913A is a "double overdrive" @ .73 in top gear. The A or B simply states the gear set, it does not mean different overdrive ratio.