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Re: driving questions

Subject: Re: driving questions
From: Chris Delling <>
Date: Thu, 27 Nov 1997 23:10:49 -0500
At 03:56 PM 11/28/97 -0500, wrote:
>As for improved gas mileage, in my simple mind I can't see how putting the
>transmission into OD will reduce the energy required to move the car down the
>road, and, since an engine is most efficient at its torque peak RPM, the
>mechanical energy required to move the car can most efficiently be converted
>from the energy content of the gasoline if the engine is operated near that
>torque peak, not at a relatively ineffecient 2500 RPM.


I will agree that the difference that overdrive makes in terms of
mechanical wear is probably negligible.  My experience, having driven both
with, and without an O.D., is that the pleasure of driving with the O.D.,
in terms of reduced noise and vibration, at freeway speeds makes it very
worthwhile.  It may be somewhat less significant in a GT, as I assume that
there is better "isolation" from the aforementioned NVH issues.

As for fuel economy, I assure you that your assumptions are wrong.
Consider efficiency.  It is simply the ratio of energy in to energy out.
As you correctly state, the engine is at it's most efficient point when
operating at it's power peak.  This does not equate to the most economy,
however, as economy and efficiency are not directly tied to one another.
Consider your car.  What throttle opening is required to achieve 60 MPH in
3rd.  Now how about 4th?  Each cylinder has a constant displacement.  If
you are filling that cylinder more completely (larger throttle opening),
and more often (higher R.P.M.) it's a given that you are using more gas.  


Chris Delling

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