Ideal cruising RPM for MP8?

I have a Mack day cab with the MP8 505HP / 1860 TQ and an 18 speed with 11R24.5 rubber. Gears are 3.79. I am in Texas and most roads I drive on are 75 MPH limit and some 80. Doing 78 MPH in 18th it is turning 1,700 RPM. I’m thinking it would be much better down around 1,300 RPM? The truck drives great and 90% of the time I gross around 65k with a flatbed. I’m thinking maybe I should regear? My fuel mileage stinks. I have never got out of the 4’s. Would getting the RPMs down help a lot? I have noticed I am in the upper half of my gears at 15 MPH. Seems I have a ton of gears down low but need a higher one. This truck isn’t on the road every day or even every week but most trips are 3-600 miles.

 

You may have a valid point. Another issue is i can’t drive a full ten hours without needing fuel. It would be really nice if I could extend my range a bit. Not sure how much a rehear costs but if I could get 6MPG that would save me $150 in fuel per 600 miles - maybe $600 per month - $7,200 per year

 

Slowing down isn’t really an option. I travel in a convoy with a bunch of pickups to job sites and they want to go the speed limit.

 

I’m no expert I’m really asking here but my observation is I have 18 gears and use half of them by the time I am going 15 mph and am turning too many RPM at highway speed. Seems like I could gear higher and have plenty of low gears? As it is now I only ever split between 17th and 18th.