GL: Fuel use

[LRZeitlin at aol.com]

Speak of coincidences. After the long discussion of the best way to measure 
fuel consumption (i.e., gallons per hour or miles per gallon) today's mail 
brought the latest copy of SCIENCE, the publication of the Amer. Assn. for the 
Advancement of Science. In it was an article entitled "The MPG Illusion." 
(Larrick, R. P. and Soll, J. B.   "The MPG Illusion", Science, Vol. 320, 20 June 
2008, pp.1953-1954) 

The authors, professors at Duke University, reported on a series of 
experiments which demonstrated a systematic misunderstanding of MPG as a measure of 
fuel efficiency. People, almost invariably, tend to undervalue the effect of 
small improvements on inefficient vehicles. For example, if asked to estimate 
which change saves the most gas, going from a car that gets 15 MPG to one that 
gets 20 MPG, from a car that gets 20 MPG to one that gets 30 MPG, or from one 
that gets 30 MPG to one that gets 44 MPG (a Prius). Most people chose the change 
form 30 MPG to 44 MPG as saving the most gas when, in fact, it saved the 
least. The big difference in MPG between the two cars gave a false illusion of gas 
saving. However if the difference in fuel consumption is expressed as gallons 
per 100 miles (GPM), the way most European do, a mistake in gas saving is 
rarely made.

I haven't had time to put this into the context of boating yet but it seems 
to me that the best metric for fuel efficiency is gallons per 100 NM rather 
than gallons per hour or miles per gallon.

As an example of the various metrics, my Willard 30 uses .82 gallons per hour 
at its cruising speed of 6 KT. That works out to 7.3 MPG or 13.7 gallons per 
100 NM. If I want to calculate the amount of fuel I will use on a given trip, 
it is far easier to use the gallons per 100 miles metric. For instance on a 
200 NM trip I will use 27.4 gallons. Crossing the Atlantic, a 3000 mile trip, 
would take 30 x 13.7 gallons or 411 gallons. Since my boat's fuel tank only 
holds 120 gallons, I guess such a trip is out of the question.

Larry Z




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