Overdrive causes your driveshaft to turn slower than your engine (normally
it turns faster to provide more torque to the wheels). When the
transmission shifts into overdrive (or you shift it in case of the manual
tranny), your engine rpms will drop (with the intention of saving fuel) and
you will lose power. By turning OD off or stomping on the pedal, you get
the rpms back, but lose your mpg.
Let's say that 1st gear in your tranny has a ratio of 2.8 (made up) and you
have 3.55 gears in your rear end. When the tranny is in 1st gear, the
engine crankshaft is revolving 9.94 times for every time the axle rotates.
This provides for maximum torque to the axle to get your vehicle moving. As
the tranny shifts thru the gears and you get to overdrive (ratio of 0.9:1 or
thereabouts), your engine will turn only 3.2 times for every revolution of
the axle. Now, to make things more complicated, the fact that your tire's
diameter is larger than the axle will cause you to lose some of that torque.
That is why small-diameter tires have the effect of increasing (numerically)
your rear end ratio, while large-diameter tires have the opposite effect.
Make sense??
-----Original Message-----
From: David Slocum <preacher@carolina.rr.com>
To: dakota-truck@buffnet.net <dakota-truck@buffnet.net>
Date: Monday, March 08, 1999 8:35 PM
Subject: DML: Overdrive
>
>
>Original Thread: Air Intake
>
>----- Original Message -----
>From: mrdancer <mrdancer@camalott.com>
>To: <dakota-truck@buffnet.net>
>Sent: Monday, March 08, 1999 8:11 PM
>Subject: DML: Re: Air Intake
>
><snipped for brevity>
>
>Can someone explain, in layman's terms ( !! ) Overdrive? What, exactly, is
>it? How does it work? How does it improve gas mileage? Why does it sound as
>if my Dak is working harder when I turn it off?
>
>I had thought that Overdrive simply forced the tranny into a lower gear on
>demand to give you a boost (like downshifting manually on a grade or to get
>the torque neccessary to pass a BigRig quicky).
>
>Preacher (Performance-engineering-impaired)
>99 5.2L CC 4x4 SLT
>http://fly.to/bastaards
>
>
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