Jesse,
You make a good point in a purely mathematical sense, but I nonetheless
completely disagree in real-world applications.
For example, the distance I drive is almost completely independent of
the car I buy. I would drive to work every morning and drive home every
evening irrespective of whether I buy the latest fuel-efficient
technology or a Silverado. But the amount of fuel consumed would vary
greatly.
In the real world, Congress has two strong tools to reduce gasoline
consumption. One would be to put a high tax on gasoline to encourage
people to drive less. Democrats shun high gasoline taxes because the
gas tax burdens poor people more than the wealthy. Republicans, on the
other hand, shun high gasoline taxes because Republicans are against tax
increases and because they don't want to be tarred and feathered. Thus,
the U.S. uses fleet averages to reduce waste without limiting distance
driven, which makes averaging volume/distance the meaningful way to
compute fleet averages.
Congress should set fleet averages in terms of volume/distance, not
distance/volume.
J.
Jesse wrote:
Thanks for the reply. I'll have to think about it more, but I don't think I
agree with this
argument. There appears to be a subtle logical flaw here. Namely, I'm not
sure the concept of
"fleet average fuel efficiency" is useful, using either form of averaging.
Averaging the km/L of several vehicles gives you the average number of miles
traveled if we
restrict all of the vehicles to use the same amount of gas. Averaging the
L/100 km (or L/Mm if
you prefer) of several vehicles gives you the average volume of gas consumed if
we restrict all of
the vehicles to travel the same distance. If the vehicles travel different
distances and use
different amounts of fuel, neither "average" would be useful in determining
either the fuel
consumed or the distance traveled, at least using any formula I can think of.
You certainly can't
just average the distance traveled and multiply by the average L/100 km. That
would give you a
meaningless number.
In almost any case in which we are dealing with vehicles with vastly different
fuel consumption, I
would think that the distances traveled per vehicle would be uncorrelated with one another.
Therefore, if "average fuel efficiency" has somehow made it into our law, then it is that useless
concept itself which should be jettisoned. And indeed the L/100 km average is
precisely as
misleading as the km/L average, though in a different way.
By all means, correct me if I'm wrong.
--- "J. Ward" <[EMAIL PROTECTED]> wrote:
The biggest disadvantage of km/L is to give a huge loop-hole to auto
maker who sell gas guzzlers. For example, the U.S. government has
requirements for fleet averages in terms of miles/gallon. Imagine I'm
an automaker and I want to sell muscle cars or monster trucks that
average 5 miles/gallon. If I can sell a single 55 mpg economy car for
every 5 mpg gas-hog I sell to get a respectable fleet average of 30
mpg. This misleads the public (and most Congressmen!) into assuming
that the cars sold use the same amount of fuel to go a given distance as
two vehicles that individually average 30 mpg. Nothing could be further
from the truth! In fact, the two vehicles use the same amount of fuel
to go a given distance as two vehicles that individually average 9 mpg.
Now work the same problem in L/100 km. The economy car uses 4.3 L/100
km. The guzzler uses 47 km/100 L. The fleet average is 26 L/100 km,
which accurately reflects the huge amount of fuel used by the fleet per
kilometer driven. Clearly, Europe is doing a lot better than we are in
terms of how they measure fuel consumption.
Finally, I would add that an SI purist would never use km/L. As a
physicist I would speak in terms of "per meter squared."
Therefore, I prefer L/100 km over km/L, clumsy as it may seem at first
glance.
J.
Bill Potts wrote:
Clearly, km/L is the more rational.
Bill Potts
Roseville, CA
http://metric1.org [SI Navigator]
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf
Of Ziser, Jesse
Sent: Sunday, January 27, 2008 17:38
To: U.S. Metric Association
Subject: [USMA:40252] Re: convenient numerical values
--- Pat Naughtin <[EMAIL PROTECTED]> wrote:
On 2008/01/28, at 8:10 AM, <[EMAIL PROTECTED]> <[EMAIL PROTECTED]> wrote:
Deciliter in the denominator is conventional medical practice in
which "convenient numerical values" are considered more valuable
than coherence of units. The same is true for grams and mg in
medical practice.
Gene.
Dear Gene and All,
The expression you use here, 'convenient numerical values' appears
quite often in many different contexts and, it seems to me, that this
is at the expense of an efficient metrication upgrade.
Another example is the change from millibars to hectopascals in
meteorology where the numbers stay the same while the unit name
changes without gaining the benefits of the coherence of the metric
system or the convenience of the 'rule of thousands'. I am sure that
there are many other examples.
I'd like to offer another possible example of violation of the rule of
thousands. I keep seeing L/100 km in fuel efficiency contexts. I also
occasionally see km/L but it appears to be rarer.
km/L is clearly more "thousandy", and also has the debatable advantage of
being "distance per volume" just like MPG. Besides, "L/100 km" seems an
awkward mouthful. Is this really the preferred unit?
I'm thinking about getting metric mileage bumper stickers for my friends and
family (most of whom I'm sure would enthusiastically accept and display
them) and I was wondering if anyone had any other opinions on the km/L
versus L/100 km issue. I've been unable to find much about it online.
Thanks.
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