What Does MPG Really Mean?
Thursday, October 16th, 2008
MPG, or miles per gallon, is a major selling point for cars, but as the website RealClimate pointed out recently, there’s a lot of confusion as to what MPG really means for consumers and for public policy.
Take RealClimate’s example:
Which driver will reduce fuel usage and emissions more?
A. The driver swapping an old 12 MPG SUV for a hybrid 18 MPG SUV
B. The driver swapping a 25 MPG compact for a new 46 MPG Prius
I subtracted 12 from 18 and got 6; then subtracted 25 from 46 and got 21 and compared the two. I assumed the correct answer is A. WRONG! The correct answer is B, the driver swapping the old SUV for a hybrid. (Of course, a driver who swaps a gas-guzzling SUV for a compact hybrid would reduce his/her own fuel usage and emissions the most.)
The mistake, which the writers assure me is common, arises because most people think of numbers linearly and assume that a reduction in MPG has a similar impact on fuel usage. It doesn’t. Assuming a driving distance of 100 miles, example B saves just 1.8 gallons (100/25 - 100/46 = 1.8), while example A saves 2.8 gallons (100/12 - 100/18 = 2.8).
This is an important point, because it demonstrates that efficiencies gained at the low end of the scale can have a much bigger impact than efficiencies gained at the higher end.
Read RealClimate’s column at: http://www.realclimate.org/index.php/archives/2008/09/the-mpg-confusion/
