Second Car Can Slash Fuel Savings, Researchers Say

Comparing it to ordering a diet soda with a side of fries, U.S. researchers say households that buy a fuel-efficient vehicle tend to buy a bigger, more powerful second car to compensate.

This, combined with the changes in driving behavior that result, may reduce the expected future gas savings from increased fuel economy in two-car households up to 60%.

A white paper from the University of California-Davis, Massachusetts Institute of Technology and Yale University estimates how the fuel economy of a newly added vehicle depends on the fuel economy of a vehicle already owned by the household.

Author David Rapson, co-director of the Davis Energy Economics Program at UC-Davis, says the results have implications for policies that improve the fuel economy of light-duty vehicles, such as fuel-economy standards.

Rapson says on average, fuel-economy standards are putting more fuel-efficient cars in households.

“That can be good if it reduces gasoline use,” he says. “But if it causes people to buy a bigger, less fuel-efficient second car to compensate, this unintended effect will erode the intended goals of the policy.”

For example, he says, consumers may prefer one SUV and one sedan, or one powerful vehicle and one fuel-efficient vehicle. “This is likely driven by practical considerations,” Rapson says.

A couple or family may drive a “gas-sipper” for most of their day-to-day tasks, but also own a bigger “gas-guzzler” for going to the dog park, camping, road trips, ski trips, the hardware store and other places where more space and power may be useful.

The study finds that, for the typical California two-car household, net savings from the increasing fuel economy of one car falls from an estimated 68 gallons (257 L) a year to 24-27 gallons (91-102 L).

Rapson says he’s not suggesting getting rid of fuel-economy standards and doing nothing else, but conservation policies need to be effective.

“A more promising solution is to put a price on carbon, such as what California is doing with cap-and-trade,” he says. “It produces economic incentives that evidence shows will reduce carbon emissions.”