Okay, here it is in a nutshell:

The lower the gear, the better performace

The higher the gear, the better the fuel economy

1st - 3rd gears = good for acceleration and passing, but bad to stay in all the time if you're looking for the best fuel economy.

That's why GM's 6 speed manuals have that 1st - 4th slip if you shift at under 2000 rpm (a pin slides out and locks 2nd and 3rd out).