I want a car that uses less gasoline — fewer gallons per mile — but car makers quote fuel economy in miles per gallon instead. The resulting numbers are counter-intuitive, leading to decisions that create needless waste, cost, and CO2 emissions.
Consider: Upgrading from 20 mpg to 35 mpg seems similar to upgrading from 35 to 50 mpg (“better by 15 mpg”), but in reality, the first upgrade saves more than twice as much fuel per trip.
To make sensible decisions as consumers, investors, and regulators, we need to think in terms of gas consumption, and that’s measured in gallons per mile, not miles per gallon. If fuel economy were measured this way, simple subtraction would give the right answer, and I’d bet that people would choose slimmer gas-hogs and use less fuel — millions and millions of gallons less, just by printing more meaningful numbers on EPA stickers at dealerships.
To avoid fractions, let’s quote consumption in gallons per hundred miles: gphm. (Europe uses liters per hundred kilometers.) Consider four cars, chosen from the best and worst of their categories as listed at www.fueleconomy.gov:
* Not on sale
Both SUVs look bad, but not greatly different, while the Wondercar looks like a vast improvement over the hybrid. These appearances are misleading, however. Here are the straightforward, right-side-up numbers:
Turning the numbers right side up clarifies the situation: Upgrading from SUV #1 to SUV #2 saves 4.7 gphm; dumping SUV #2 in favor of the best hybrid would save an additional 2.2 gphm. Compared to the hybrid, the Wondercar saves 1.1 gphm. Going straight from SUV #2, to the Wondercar would save a total of 3.3 gphm, less than the savings of switching from SUV #1 to #2. (By the way, the best and worst SUVs are both Jeeps, the Grand Cherokee and the Patriot; the hybrid is the Toyota Prius.)
Discussions of fuel conservation would make more sense if we used units that made more sense. Let’s not talk about mpg: it’s confusing, and trading up to gallons per hundred miles costs nothing. Maybe actual gas consumption figures should be on EPA car stickers at dealerships, too.
16 March update: Here’s an article from Duke University on the advantages of using sensible units: Gallons Per Mile Makes More Sense. I’m pretty sure it was this work, reported last June, that prompted me to write on this topic.
14/04/24: Cleaned up numbers and description in first example.