RPM at 70MPH

To help with selecting a happy cruise rpm, I do this;
With the engine fully warmed up, and the timing set right/bugged out,and the tranny in Neutral, and a vacuum gauge hooked to manifold vacuum; I slowly bring the rpms up, while watching the gauge. The lowest rpm that makes the highest vacuum I consider the lowest happy cruise rpm. Happy, in the sense that the engine is finally out of reversion, and becoming efficient.
Now that I know this,I take it out for a cruise. I get it into high gear. I start at 45ish mph, on a flat level hi-way, on a windless day.After the mph becomes stable, I mark down the speed and vacuum reading. I continue in 5mph increments. Sooner or later the engine vacuum will peak at a high number, and sooner or later the vacuum will begin to drop, as she works harder to overcome wind resistance and chassis friction. Somewhere between that maximum peak vacuum and the point at which the vacuum begins to drop, is the happy place.If I was to then marry the two,with gears, I will have a vehicle that has the best potential for fuel mileage.

-Remember the first test was to find the lowest happy rpm, while the second was to find the highest rpm.
With a stock Magnum, this rpm could be pretty low. I had a 92 TB-injected-teener Dakota that was very "happy" at 2200@70. This was with 3.91s, the A-518, and 28" tires.In a lighter chassis, I'm sure it woulda been happy at a much lower rpm.
With a hemi, the same principals still apply. But since it is an inherently better design, I wonder if the minimum cruise rpm will come in even lower.

Performance is another story.
But with the O/D tranny, You can run almost any rear-gear.

Personally I run a 367cid with 3.55s and a GV overdrive. I runs 2400@70. I find it a little busy, especially on long trips. 3.23s would bring it down to 2200 which by the above tests would be the minimum recommended for my combo, cuz that's where my combo vacuum peaks. And my combo would pull 3.23s ok as well. And I have a chunk ready to go. I just rarely cruise at more than 60, and the 367 is very peppy with 3.55s, and mostly, I'm getting lazy in my old age.heehee