Deeper gears less effective ?

Im not sure I understand the point, but I know this:
The last two gear changes I made on two different cars (from 4.56 to 3.91, bbm, and from 5.14 to 4.57, n/a small block) the cars ran virtually the same et and mph.
Both cars had plenty of stall speed, that might have been a mitigating factor.
But I'm still changing my current car from 3.50 to 4.10.