What rpm at 65mph ???

I have 3.55 in my truck with 1122.5 and I cruise in 8th direct usually, 62-64ish at 1650-1700, lugging a motor down below 15-1400 rpms does nothing but build heat and cause wear and tear,

 

Depends on the motor, doesn't it?

 

Yes. I wouldn’t want to run a MX13 at that rpm all day long.

 

The 6-71 was only 426 cid, gutless.

Regardless of engine brand, as long as you're on the flat part of the torque curve you're still getting economy as long as you have a full load on.
Running empty or light at higher rpms is when you're just wasting fuel.

It's when the torque drops that the potential fuel efficiency drops, in the engine below you're still getting reasonable exonomy at 1700 rpm.

 

Let me know what you get done with an 855 or 3406 running like that, Egts and oil temp stay a lot cooler on my n14 running ~1600 then 1400, heat kills more then revs,

the last company truck I had with a paccar had 3.90s (1124.5) and was getting over 6 hauling well over 100k, would turn about 1600 at 70 and was really geared perfectly for that.

 

I'll second that with my b model. 1450 is fine cruising on the flat when I'm empty. Any other time, its 1550 or higher. But we're obviously talking about antiquated stuff... nobody runs big cams or mechanical cats anymore.