
The Chevy Volt Gets 230 mpg? Only if you use bad math. - Flemlord
http://scienceblogs.com/goodmath/2009/08/the_chevy_volt_gets_230_mpg_on.php
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JimmyL
Miles per gallon answers a question no one asks - you don't say "I have a
gallon of gas, how far can I go?", you ask "I am going 40 miles, how much gas
will it take?" MPG comparisons are based on two vehicles using the same amount
of gas and seeing which will go farther, which clearly isn't how people think.
Gal/100mi are based on cars driving the same distance and seeing which uses
less gas, which is more applicable to real habits.

There's also an interesting study done by some guys at Duke (summarized at
<http://bit.ly/mYpFV>) showing how GPM makes it much easier for people to
grasp fuel savings than MPG does. Let's say I have two cars, each of which is
driven 10K miles per year - not unreasonable for a suburban household. One of
my cars gets 35MPG, and one gets 20MPG. I can replace the former with a 50MPG
car, or the latter with a 30MPG - for differences of 15 and 10MPG
respectively. most people would pick the former.

But in that case, the 15MPG increase would only save 85gal, whereas the 10MPG
increase would save 160gal. If you flip the numbers, however, and consider the
choices as Gallons per 100mi, it becomes much clearer:

    
    
      MPG    Gal/100mi  Savings (gal/100mi)   Savings (MPG)
      
      35     2.85
      50     2.00       0.85 gal/100mi        15
      
      20     5.00
      30     3.33       1.66 gal/100mi        10
    

The 20 -> 30MPG switch is empirically a better deal, all else equal (since we
don't expect our driving habits to change with the new car). Gal/100mi sure
makes it easier to see that, though.

------
jws
The rant is sort of on, but not really. MPG is a bad name, but it is based on
the EPA's draft standard for plugin hybrids. What it is telling you is that
for a typical city driver who charges up each night, by the time the odometer
reads 230 miles they will have used one gallon of gas. They also are kind
enough to report that the car gets 50 mpg when running the engine, so other
than the headline they aren't being too deceitful.

I'm not sure what the EPA stickers will look like, but I hope they have "kwhr
per mile (electric)" and "gallons per mile (gas)" as well as the standard
numbers. It is time to fix our silly reciprocal measurement (mpg is silly. No
one says "I'm gonna burn me a gallon of gas and see how far I can go!")

~~~
tb
In metric countries (i.e. most of the rest of the world) we use litres per
100km, which is the "right" way around. :)

