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MPG vs. GPM vs. percentages Thu, Sep 04, 2008
Mental Floss wrote an article (referencing this Treehugger article) about how gas mileage should be measured in gallons per mile instead of miles per gallon.  The argument is that the metric used to measure a car's gas usage efficiency should be gallons instead of miles, because it's easier to see a direct correlation to cost and energy savings.  For example, increasing your gas mileage from 10 to 15 mpg means you can travel 5 more miles for each gallon of gas you would normally use.  But that fact isn't all that useful to normal people.  Great, I can travel more miles.  But how much money am I saving?  Taking the inverse (1/10 and 1/15), your vehicle's gas usage is decreasing from 0.1 gpm to 0.067 gpm.  Multiply it by a typical number of miles you drive (say 1000 miles per month), and that equates to a savings of 100 - 66.67 = 33.33 gallons per month.  Take whatever the price of gas is at the moment (maybe $3.50), and you can easily see that you can save $116.66 per month if you switch from a vehicle that gets 10 mpg (or 0.1 gpm) to a vehicle that gets 15 mpg (or 0.067 gpm). 

However, I took things a step further and put everything in terms of percentages.  It's much more useful to see how much something has changed when it's compared to the original. 

mpg 1 mpg 2 mpg change gpm 1 gpm 2 gpm change
10 15 50% 0.1 0.067 -33%
15 20 33% 0.067 0.05 -25%
30 40 33% 0.033 0.025 -25%
30 45 50% 0.033 0.02 -33%
40 50 25% 0.025 0.02 -20%

In plain English, the table says, "Going from [mpg 1] to [mpg 2] means a [mpg change] increase in the number of miles you can drive, or a [gpm change] decrease in the amount of gas you'd use."  The values for mpg I used were intentional.  The first example essentially proves why the Chevy Tahoe Hybrid is the Green Car of the Year.  Going from 10 to 15 mpg (essentially what the Tahoe did) is a 50% increase in gas mileage, or a 33% decrease in gas usage.  That's incredible.  It's the equivalent of going from 30 mpg (a standard "good" sedan gas mileage) to 45 mpg (a standard hybrid sedan gas mileage).  Also noteworthy is the fact that an increase from 15 to 20 mpg is equivalent to an increase from 30 to 40 mpg.  Finally, the jump from 40 to 50 mpg (a magic number for car makers) equates to half the mileage increase as the 10 to 15 jump, or only 60% of the gas savings. 

Despite all this wonderful math, the problem with this way of thinking is that percentages are relative (I think that's a math joke; please don't laugh).  To say that something increased 50% from what it originally was ignores the fact that it may have originally been quite low.  Such is the case with gas mileage.  Sure, going from 10 to 15 mpg is a 50% increase in gas mileage, but a 30 mpg vehicle is already using 67% less gas over the same exact distance.  So of course it's good to get better.  But it helps when you're already doing pretty well. #travel


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