From: John De Armond
Newsgroups: misc.consumers.frugal-living
Subject: Re: Driving a 5 speed to get the most MPG's
Date: Wed, 28 Sep 2005 15:13:55 -0400
Message-ID: <vvplj1tob1mvsfsj2782otufrbsl57ohc1@4ax.com>

On Wed, 28 Sep 2005 08:10:58 GMT, "Kat" <nospam@nospam.com> wrote:

>With-in the week I will be buying a manual transmission car.
>Do you have any tips how to get the most MPG's?  I know how to drive a
>stick, but don't necessarily know how to drive it to insure that I make the
>most of the mileage.
>Thanks,
>

You probably won't see any significant improvement in mileage over an
auto.  There are several reasons.  First, the electronic controls have
gotten so good that the computer can generally do a better job than a
human.  Second, lockup torque converters have pretty much eliminated
the major source of loss in the auto drivetrain.  Third, the added
control afforded the electronics by controlling the shifting gives the
OEM tuners more latitude when tuning the engine for efficiency while
still meeting emission standards.  Forth, the usual differences on the
EPA sticker are artifacts of the rather artificial standard driving
cycle specified by the EPA for emission tests.

If you want the best economy you'll have to do some testing and
careful record-keeping.  In general, one wants to keep the engine RPM
as low as possible to minimize frictional losses and the intake
manifold pressure as high as possible to minimize pumping losses.  The
later is pretty much the opposite of the conventional wisdom for
carbureted engines, to keep the manifold vacuum as high as possible.
This had more to do with the limitations of the carburetor than engine
condition.

The objective will be to keep the engine operating as close to its
peak efficiency point.  This is generally low speed and near wide open
throttle.  The single exception to wide open throttle is if the engine
control has a wide-open-throttle enrichment mode.  Most modern cars
don't, though some of the early EFI'd cars did.

Without some rather expensive test equipment, the way to find this
point is to drive the same route over and over, maintaining known
settings (speed, throttle position) as much as possible and carefully
recording gas consumption.  Modern EFI'd engines have a fairly broad
efficiency range so don't expect to see a lot of differences.

Of much more importance is to find the knee of the power vs speed
curve.  The power required and therefore fuel consumption goes up as
the cube of speed.  At some point there is a sharp knee in the fuel
consumption vs speed curve, as the fuel consumption skyrockets.  This
is the basis of the simplistic chant "drive 55".

The knee location varies widely by body style.  My diesel cube van
hits the knee at about 58 mph while my car hits it at about 77.  The
way to discover the location of the knee is to drive the same route at
different speeds over a long enough distance to consume several
gallons of fuel and carefully record the mileage.  You'll probably
find that only 2-3 mph change in speed makes a large difference around
the knee.

John