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Re: Fuel efficiency versus speed



Dear Laurent,

I don't know of any current data for trucks. However, since air
resistance goes roughly as the square of the speed, and on the open highway
most of the net horsepower (after thermal losses) goes to balance the force
of air resistance there isn't any way that you win on mpg by going faster.

Mark Shapiro
http://www.IrascibleProfessor.com


-----Original Message-----
From: Laurent Hodges [mailto:lhodges@IASTATE.EDU]
Sent: Friday, October 06, 2000 9:13 AM
To: PHYS-L@lists.nau.edu
Subject: Fuel efficiency versus speed


Does anyone know a good source for fuel efficiency versus speed data for
various types of vehicles, in
either tabular or graphical form?

I haven't seen this on the internet, or anywhere in recent years. Such
information was available back 25 years ago, during the energy crisis of
the 1970s.

In particular, truck drivers were insisting that the 55 mph speed limit
actually hurt their fuel economy, lowering their miles per gallon. They
had a lot of strange reasons for thinking this, and were constantly
criticizing various theoretical and experimental studies on tracks,
claiming a real journey with real trucks on highways with ups and downs and
curves and the like would prove their point. ("Physicists don't know
anything about driving trucks!") They managed to get a major federal study
carried out in which identical real trucks with real cargos were sent on a
real journey all around Virginia (I believe), hills and flat parts and all.
Some of the trucks were to go as fast as possible, but no faster than 70
mph, or 60 mph, or 50 mph, etc. The result: the greatest fuel economy
(highest miles per gallon) occurred for the lowest maximum speed, which was
20 or 30 mph. The experimental curve matched theoretical expectations, of
course. The truckers who were so enthusiastic about this study ahead of
time did not broadcast the results!

Laurent Hodges, Professor of Physics
12 Physics Hall, Iowa State University, Ames, IA 50011-3160
lhodges@iastate.edu http://www.public.iastate.edu/~lhodges