GreenCarReports.com wrote a story on this over a year ago, and just this week the National Research Council has come out with a report that basically agrees – mpg is a poor way to measure a car’s gasoline use. Basically, it comes down to this – would you save more gasoline by going from 10 to 20 mpg or from 33 to 50 mpg? Most people would say 33 to 50 mpg, but that’s the wrong answer. It just points out the use of miles per gallon is a bad metric if you really want to reduce fuel consumption. A far better measurement would be the volume of fuel used per 100 miles (or 1000 miles). In the first example (10 to 20 mpg), you would double your fuel efficiency (10 gallons for 100 miles @ 10 mpg, vs 5 gallons for 100 miles @ 20 mpg) for a savings of 5 gallons of gasoline. But with an improvement from 33 to 50 mpg, you would only save a little over 1 gallon for that distance.
While of course the vehicle that gets 50 mpg will use less gasoline than a vehicle getting 20 mpg, that’s not the point. Many people need large vehicles (for work, large families, etc), and mpg figures only tell part of the story. Measurements should enable consumers to make educated decisions. As the National Research Council noted in its report, the use of mpg as the only measurement “cause(s) consumers to undervalue small increases (1-4 mpg) in fuel economy for vehicles in the 15-30 mpg range.” Another interesting note is that mpg as the sole measurement is the practice only in North America. The rest of the world has moved to fuel consumed in addition to mpg.
And, not to forget, things will get even trickier with plug-in hybrids.