Overdrive, what is it?
As simple as I can describe it, 5 speed auto has one more available gear ratio than a 4 speed. A 5 speed auto can also have an overdrive. Numerically a gear ratio below 1:1, for instance .50:1 is an overdrive ratio. So for a .50:1 ratio, that means that for every one engine rotation, the transmission has gone through two full rotation. This advantage allows better fuel economy if matched at the correct vehicle speed range.
One is a slushbox, the other is a slushbox with a button