I had to pick this part out to ask....
Are you referring to the condition where a car is operated at a speed below the RPMs where the camshaft becomes efficient?
If so, I understand the principle.
I'm going with a 5 speed in my '70 Charger. The OD ratio is .64. Some have asked me why I didn't opt for the 6 speed but to me, a .050 OD gear would only serve to allow a higher top speed. Rolling along at 75 mph at 1800 rpms with a 440/493 and a solid lifter MP 528 cam seems like I'd be well under the point of efficiency and probably get the same freeway mileage as a .64 OD gear and 2200 rpms.