Re: Fuel efficiency versus speed

[Larry Cartwright <exit60@IA4U.NET>]

brian whatcott wrote:
> There should be a proviso in there that the speed to engage top or
> overdrive is likely to provide best mpg.

This was the gripe of the big-rig truckers when the expressway speed
limit was abruptly lowered in the 1970s.  Engineering big truck fuel
economy is much more complex than just lowering speed and lowering air
friction.  It is not a simple textbook problem.

1970's long-haul powertrains were geared to provide optimum engine rpm's
and torque for economy at 70 mph.  Slowing down to 55 mph forced them
into a lower gear ratio, where most of them wound up using as much fuel
as ever even with the lower speed.  *Theoretically* the decreased air
friction should have resulted in fuel economy,  but that wasn't the
actual outcome for most of the truckers.

They felt they were political victims of a poorly conceived strategy,
forced to choose between spending more time on the road or doing less
business, and they weren't using any less fuel than they ever did and it
cost more.  There was an alternative: spend thousands up front to
rebuild your rig with a new "Jimmy Carter powertrain" that would run
efficiently at 55 mph.  Big outfits retooled their fleets.  A lot of
little guys were forced out of the business.

Best wishes,

Larry
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Larry Cartwright  <exit60@ia4u.net>
Physics and Physical Science Teacher
Charlotte HS, Charlotte MI USA
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