MPG vs GPM

[careytilden]

I heard a story on NPR the other day which, at first, made me incredulous. It was discussing MPG vs GPM as a useful measure of fuel efficiency. It stated that a smaller increase in MPG at the low end--say, 10 MPG to 15 MPG--was worth more than a bigger increase at the high end--say, 30 MPG to 50 MPG.

Intuitively, this seems ridiculous. But once you do a little bit of math, it makes more sense. The most useful measure of efficiency is how much gas it takes to get from point A to point B. That is, you have a set distance, and you want to compare the amount of fuel used. That leads to GPM as a more direct measure. Since GPM is the reciprocal of MPG, it has a non-linear relationship. This throws off all our intuitive responses and makes MPG somewhat deceptive.

Here's a graph to demonstrate the relationship:



And an example to demonstrate the point:

• 10 MPG uses 10 gallons per 100 miles, 15 MPG uses 6.7 gallons per 100 miles. Moving from 10 MPG to 15 MPG saves 3.3 gallons for a 100 mile trip.
• 30 MPG uses 3.3 gallons per 100 miles, 50 MPG uses 2 gallons per 100 miles. Moving from 30 MPG to 50 MPG saves 1.3 gallons for a 100 mile trip.