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Re: 300 miles 70 mph 30 mpg :)

To: (Mark Jurras)
Subject: Re: 300 miles 70 mph 30 mpg :)
From: (Russ Wilson)
Date: Wed, 20 Dec 1995 09:23:44 -0800
Mark Jurras asked:

>Or so you thought. This morning the daily lab discussion was about
>overdrive equiped cars. The question is: if you drive a car at 45 MPH
>in 4th no O/D and calculate the Mileage then drive the same course and
>conditions at the same RPM in 4th + O/D and calculate the Mileage,
>which is better 45 no O/D or faster with O/D.

The only real advantage of overdrive (to the car - it might offer some
additional advantage to your ears) is that it reduces engine wear.  Fuel
comsumption is not the issue.  Consumption should be reduced a smidgon due
to reduced internal losses in the engine (fewer revolutions per mile), so
at the same *vehicle* speed, the car in overdrive should show better
mileage, but in my experience, this is barely noticable, if at all.

But Mark asked the question about driving at the same engine speed, ie, at
a higher vehicle speed in O/D.   Far more significant than the reduction in
engine losses is the increase in air drag at increased speeds.  As I
recall, drag increases at a rate between the second and third power of the
air speed, depending on the shape of the body.  So a 20 percent increase in
speed (this is about the typical overdrive ratio), drag, which represents a
major chunk of the work a car engine does at highway speeds, could go up 40
to 70 percent, overwhelming the savings due to reduced engine losses.

But then, why shift into O/D and keep the engine speed the same?  Let it,
and the engine noise, drop and enjoy the wind noise more.  (Yeah, right!  I
wear those little foam ear plugs if I'm going far with the top down.)


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