OP, read this again. I've seen this many times - all you get from going too deep is more engine noise and worse fuel mileage.
My big-cam 451, 4-speed, 295/50R15 tires, likes the 3.91 for mixed use. 3050 rpm at 60 mph is tolerable since it's not daily driven, and I can live with 9 mpg highway. And the cam is just coming on at 3k.
As has already been said, it all depends on the intended use of the car, the engine, the cam... no one right answer.
