got a question here. some cars (mainly american) have this "overdrive" button usually on the gear knob. what exactly does it do ?
from what i umderstand, it cuts off the final gear (i.e in a 5 speed tranny, it will only permit you to shift to 4th) and it kinda holds shifts longer. is that correct ?

and generally speaking, in automatic gearboxes, is it possible for the gearbox to have variable gear ratios ? does D3 mean longer gear ratios than shifting to D4 ?
or am i mixing gear ratio with holding a gear ?