There have been a number of comments on my original question (see down
below). Most people automatically assume lower rpm means better efficiency
(better mpg). That is not always correct.
Mike has the real answer. It requires a certain amount of horsepower to
move a given auto at a given speed down a flat road (with no head or tail
wind). For a Roadster body, I have no idea what hp is required at 70 mph
but let's assume it is 30 hp. (The real number depends on aerodynamics and
internal friction.)
So the real question is: At which rpm (3,200 or 4,000) does the engine
produce 30 hp most efficiently.
And the answer is: Beats the hell outta me. I don't have any engine
design info that I can use to get the answer. Some have commented that
changing axle ratios and/or tire sizes will change the equation. That is
correct because changes the 2 rpm points and the efficiency will be
different at those rpm.
Mike is also correct that an Otto Cycle engine (the lumps we have in our
Roadsters)(gas not diesel) is most efficient at full throttle. This way
there are fewer losses in the throttle area of the carbs. Stationary
engines are always designed to run at full throttle if possible.
Dick Ryan ex. 24022
(650)312-4022
Risk Management Dept.
SM 2000/2
FAX (650)312-5830
From: datsunmike [mailto:datsunmike@nyc.rr.com]
According to an article I read regarding MPG and engine efficiency you may
not get better gas mileage in 5th at the same highway speed. It has to do
with cams and breathing and operating the engine at optimum efficiency which
is usually close to peak torque. If the car is lugging around in 5th it may
get better gas mileage in 4th where the engine is at its most efficient RPM.
Several years ago BMW came to the conclusion after extensive testing that
the best way to get the most MPG in city driving is to accelerate with the
go pedal fully depressed and short shifting rather than gradual acceleration
as most of us do. The idea is to keep the butterflies fully opened, the same
way a diesel operates. It's the WOT that brings peak efficiency to the
engine and allows full breathing.
Mike
> From: Ryan, Richard <RRyan@frk.com>
> >With everyone talking about MPG I have a question.
> >
> >Assume you have a 2000 Roadster and you are driving on a flat freeway at
70
> >mph.
> >
> >Which gets better gas mileage?
> >
> >A. Fourth gear at 4,000 rpm
> >
> >B. Fifth gear at 3,200 rpm
> >
> >(I think these rpm estimates are about right. I haven't driven mine in
> >years.)
> >
> >
> >Why?
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