[alfa] re: Fuel economy vs. RPM

[Joe Elliott <jee@xxxxxxxxxxxxxx>]

It's a reasonable rule of thumb that a higher gear will yield lower 
fuel consumption.

In "Sports Car and Competition Driving," Paul Frere writes:
"If minimum consumption is to be achieved you must remember that, in 
a gasoline engine, the specific consumption decreases as the load on 
the engine is increased.  This means that under given conditions and 
for a given speed of the car, the engine uses less gasoline when it 
runs comparatively slowly--say in top gear--with a wide throttle 
opening than when it runs at a smaller opening and at higher 
revolutions in a lower gear.  It follows that to achieve the minimum 
fuel consumption, the engine must always be kept at the lowest 
possible revolution rate without, however, letting it drop into a 
range where efficiency is impaired.  In this connection, it must be 
remembered that an engine's most efficient range of operation is the 
one over which it produces its maximum torque.  Below this range the 
efficiency falls off and the specific consumption rises, because the 
valve timing is not adapted to such low revolutions and losses occur 
due to scavenging of unburnt mixture and inadequate filling of the 
cylinders.
"So, if maximum economy is the aim, gear changes must be performed in 
such a way that the engine is kept working under a fairly high load 
in the range where it produces its maximum torque, provided this does 
not result in too high a maximum speed, when the increase in drag 
would offset the engine's low specific consumption."

Hope that helps,
Joe Elliott


At 5:21 PM +0000 4/28/04, alfa-digest wrote:

> Date: Wed, 28 Apr 2004 08:37:43 -0500
> From: "Andy Menapace" <Andy.Menapace@domain.elided>
> Subject: [alfa] Fuel economy vs. RPM
>
> Maybe this is elementary, but:  At what RPM is the two liter motor most
> efficient?  Asked another way: At 50 MPH should I drive in 4th at 4000
> RPM or 5th at 3200 RPM?  Those numbers are rough estimates, but you get
> the idea.
>
> Just curious,
>
> Andy Menapace
> Brandon, MS
> 79 Spider
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