> wrote in message
oups.com...
> Hi all:
>
> I am new to this group and this is my first post. (I own a 2000 Ford
> explorer sport with the most basic no frill engine.)
> Here the very newbie question: What purpose does the O/D serve?
> Should I leave it on or off?
> Someone told me that leaving the O/D on was essential for highway
> driving but a waste of fuel for city driving. Is this true?
> Does anyone actually know how the O/D works? Txs.
>

For decades, all most all normal transmissions (automatic or manual) highest
gears were 1 to 1. That means that the output shaft turns 1 revolution for
every 1 revolution of the input shaft. Now it is uncommon to find a vehicle
that does not have overdrive.

Transmissions with "overdrive" have a higher (taller) gear, like .76 to 1.
That means that the output shaft turns 1 revolution for every .76
revolutions of the input shaft. That gear is over driven. The engine spins
fewer RPMs at the same travel speed, and torque is greatly reduced. For
almost all but the most extreme driving conditions overdrive results in
better fuel mileage.

I assume your transmission is an automatic? Generally the only time you
should have to be concerned about taking your transmission out of
"overdrive" would be if you were towing or hauling at or above you vehicles
GCWR Gross Combined Weight Rating. If your transmission is continually
hunting for a gear, shifting in and out of overdrive, you could use that as
an indicator to leave overdrive off.