ok, i went to do a search on overdrive and this is what i
got:
Drive in high gear
(overdrive)
The engine runs most efficiently between around 1,500 and 2,500
rpm. To maintain these low revs you should change up through the
gears as soon as practical and before the revs reach 2500
rpm.
For automatic transmission cars, you should always switch on your
overdrive to help save fuel. Overdrive will allow your engine to
change gears at lower revs. It also puts your transmission into an
"economy" mode and lets it engage the final "overdrive" gear when
cruising to keep the rpms extra low, thereby increasing fuel
economy.
