As Jessica mentioned last week, Consumer Reports recently claimed that EPA’s vehicle ratings routinely overstate how fuel-efficient cars and trucks are in real-world driving. For standard cars and trucks, the magazine says, EPA’s ratings overstate real-world fuel economy by 30 percent. But for small hybrids, such as the Toyota Prius, they claim that EPA overstates actual miles-per-gallon by a hefty 42 percent. (Ouch.)

Now, I believe that there’s reason to question Consumer Reports’ figures. Of course, I have read a number of reports that the Toyota Prius doesn’t actually get the EPA-rated 55 mpg in combined city/highway driving (though some people—particularly those who’ve optimized their hybrid-driving habits—get pretty close, and these folks actually squeezed out 110 mpg from their Prius, albeit in highly non-standard driving conditions). But I’d never heard any claim that the typical Prius averages just 32 mpg—which is what the magazine’s figures suggest. See this comment by WorldChanging’s Jamais Cascio for a similar take.

But, just for the sake of argument, let’s take the CR figures at face value, and assume that small hybrids’ mileage really is overstated by 42 percent, vs. just 30 percent for regular cars. Doesn’t the higher mpg reduction for hybrids suggest that their fuel-savings advantages vs. regular cars are overstated—and that they don’t save as much money as advertised?

Actually, no. As counterintuitive as it may sound, the Consumer Reports figures, on their face, actually bolster the economic case for buying hybrids.

  • As we’ve said before, mpg math is tricky. And though it may be hard to believe at first, Consumer Report’s figures suggest that the Prius is an even better choice in the real world than EPA’s fuel economy ratings would suggest.

    Consider, for example, a regular, non-hybrid car with an EPA fuel economy rating of 30 mpg. “Officially” it burns 2 gallons of gas every 60 miles. But Consumer Reports estimates that the vehicle actually would get 21 mpg in real world driving—30 percent less than advertised. Which means that over the course of 60 miles, the car actually burns 2.9 gallons of gas.

    Now consider the Prius, with an overall EPA fuel economy rating of 55 mpg. At its advertised mileage, it burns about 1.1 gallons of gas every 60 miles. But if its mileage is reduced to 32 mpg—a 42 percent reduction, per Consumer Reports’ estimate for small hybrids—then it uses about 1.9 gallons of gas every 60 miles.

    So that gives us…

    Gallons consumed in 60 miles
    “Ideal” “Real World”
    Prius (rated 55 mpg) 1.1 1.9
    Regular car (rated 30 mpg) 2 2.9

    Look at the “ideal” column—which represents how much gas EPA says the two cars should burn over 60 miles. The Prius has a .9 gallon advantage—a nice bonus. But look at the “real world” column: as estimated using Consumer Reports’ figures, the “real world” Prius has a full one-gallon advantage over the “real world” regular car.

    In other words, the Prius is actually an even better deal—roughly 10 percent better—in the real world than it is in the abstract. And compared with lower-mileage cars and trucks, the “real world” Prius looks better still.

    Now, this obviously isn’t evidence for or against Consumer Reports’ estimates for hybrids. And it doesn’t do much to change myassessment that buying a Prius can be a pretty pricey way to reduce your greenhouse gas emissions.

    Still, it’s good to remember that, as Barbie famously said, math is hard—and miles-per-gallon math turns out to be among the more counterintuitive gauges that Americans are expected to understand. So it’s important to actually run the numbers. Apparently, at least when it comes to gas mileage, it’s just not enough to trust your instincts.