Not getting into the technical details of the internal workings of the
transmission, the difference is the rate at which the rear wheels will turn
in relation to engine speed. This is known as final drive ratio. In OD,
the rear wheels turn slightly faster for the same engine RPM as in Drive.
There are a few ways that this is done - an extra gear, a lock-up torque
converter, etc. Others here are more qualified than I if you want a
detailed explanation. Depending on the set-up of the drive train, OD may
also lower your towing/hauling capacity, but may increase your top speed and
MPG while unloaded.
Cheers - Jonathan
Close, basically it is another gear after drive. Drive is 1 to 1 (one
engine rev to one output shaft rev) and OD on a automatic is usually in
the .7 to .75 to 1 range (dependend on brand) so engine turns 25 to 30%
percent slower when it is engaged in a effort to save a little fuel in
most cases when vehical is lightly loaded and on fairly flat ground.
Overdrive means the final gear ration is lower than 1/1 ratio. A OD tranny
generally gives you better fuel mileage because you run the engine at a
lower RPM. Early chevy OD transmissions were somewhat problematic but todays
4L60 is a dependable beast and when you do have problems its a cheap and
easy rebuild. if you can avoid 87 and earlier 4l60 and 700r4. Also avoid
towing in OD because the 2/4 band won't stand up to it, if you tow simple
pull it down to drive.
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