The biggest issue might be the cam.
A performance cam may still be in reversion well below 2000 rpm. That means the pistons are pushing mixture back up into the intake, on their way back up, until the intake valve finally closes.
This causes three things; firstly this reduces your effective compression ratio. Not your Static nor your Dynamic, but your Effective Cr.Which reduces your torque. Perhaps to a point where any little irregularity in terrain or wind, will require constant throttle manipulation to maintain speed.
Second; is tuning;It's hard to get the AFR lean enough under these conditions to reduce fuel consumption from that angle.
And third, if you have to run it rich to compensate for the falling efficiency, what are the ramifications of that? Increased fuel consumption, unburned fuel in the oil pan and fuel in the exhaust system;non of which is saving you money.
Every engine has a minimum rpm as far as reversion is concerned. To find it, I put a vacuum gauge on it and slowly rev it up. The lowest rpm at which the vacuum peaks is the first time reversion has stopped. I would consider that a "target"cruise rpm.
This is not a hard and fast rule tho, because the engine will continue to use less fuel with less rpm, even tho it is operating in reversion. But there will be a break-even point somewhere where any lower rpm will consume more fuel; and in the meantime the Afr will be a bit of a challenge.
So in answer to your question, yes.
I have akindof rule of thumb guideline; whatever your cam measures at .050, multiply it by 10 and that is my target minimum cruise rpm. Therefore a 200 cam works out to 2000, and a 240cam looks like 2400. And yes that is the target, not the absolute. The vacuum gauge is another good method, and is more engine specific.
________________________________________________
But I'm a little stumped why, with an overdrive, you might want to run that low a ratio.
Most of us run OD because we want the lower starter gear, to get moving quicker, or we want to optimize second gear, or some other rpm or speed. We usually just want the od to get us a lower cruise rpm than the 65=3000 we might be currently running;ANYTHING lower,lol. And so 3.91s might be the go-to gear. These drop the Rs at 65 in od, to about 2200, and all my SBMs have been quite happy at that rpm, which is about a 2.70 final drive.
Altho I had this one Hi-compression, small cam 367, that was Ok with a 2.02 final-drive, making incredible mileage doing about 2000@80mph. Yeah that one ran 10.9Scr and around 185psi. That one was actually pretty good down to 60=1400;also in double od.
So if you build your engine to make big VP down there, it will reward you with excellent fuel-economy; whether or not it will make power at 6000 is another story.
