Funny, the truck has the original speedometer - the only change in the drive line that I've made (so far) is installing taller (slightly) tires and 16" wheels.
I've passed by several school zones with radar activated speed sensors to alert drivers of their speed in the school zone.
Result? The speedometer is 2 MPH faster at most speed points (?). It should be slower.

The 5 speed 5th gear is only .78 to 1 overdrive. Maybe reduce by 500 RPMs ? Better than nothing, but 1000 would be better. Still in the torque zone to not lug the engine.

I usually rent vehicles when I'm on assignment. Most of the new vehicles run about 2000 RPM at 80 MPH!!!
I know they are a totally different banana, but I do a sht ton of highway driving and need lower revs.
I'm not as concerned with acceleration, although I don't want a total slug either.

Anyway, one step at a time.