A popular criticism of hybrid and fuel efficient cars goes like this: If your car is more fuel efficient, and you know you're saving money on gas, you'll be more inclined to drive more—thus negating any energy and carbon savings you had.
Called the 'Prius fallacy', it's a variation of the 'rebound effect', which posits that any energy savings we gain from more efficient technologies will be promptly negated by our using more energy (either elsewhere, or by doing more of the energy-consuming activity). And while the rebound effect is a real phenomenon, and does eat away energy savings in some cases, the Prius fallacy has always struck me as fundamentally fishy. The CO2 Scorecard defines its two basic tenets:"1) that Prius drivers drive more because they are paying less in gas, and/or (2) that Prius drivers use money saved on fuel to purchase or participate in energy- & carbon-intensive goods and activities"
Now, most people use their cars to commute and to run errands—it always seemed weird to me to argue that if your car was suddenly cheaper to refuel, you'd suddenly find more places to drive to, just because it wasn't straining your wallet as much. How many Prius drivers do you know that go joy-riding, just for the hell of it, that wouldn't have done so before with gas-guzzling cars? Where do these fictive fallacy-makers go that they weren't going to before?
It turns out that my gut reaction was reasonably well-founded. CO2 Scorecard has crunched the numbers, and they found that the Prius Fallacy is, well, fallacious. From their report:
The result obliterates the Prius Fallacy’s first assumption. As shown in the comparative histogram in Exhibit-2, there is no difference in VMT by Prius owners and the rest of California’s drivers. On average Prius owners drove 13,130 VMT/year compared to 13,064 VMT/year for non-Prius owners—a difference of a mere 0.5%.In fact, if you switch from an SUV to a Prius, according to CS, you'd save 430 gallons of gas a year, factoring in that additional rounding error that Prius owners drive above car owners.
As for the second prong, CS concludes (read their full analysis here) that Prius owners aren't spending the money they save on gas much differently than money any of save on anything else—sure some of it goes towards carbon-intensive activities, but that fraction is hardly enough to come anywhere close to negating the energy savings earned by the fuel efficient auto.