If you own a Prius, it's likely you're also going to own a gas guzzler too: UC Davis study

Prius (left) and BMW SUV X5 (right) 

Californians who have high-fuel efficiency vehicles – like a Prius – also tend to choose a gas-guzzler as a second car of choice, according to a new University of California at Davis study.

The study’s authors liken the trade-off in fuel economy to the “diet soda effect,” people who consume diet drinks and then use that decision to justify eating high-calorie foods.

In car terms, that means if drivers own a Tesla, then they might feel it’s OK if they also own a BMW X-5 SUV  for weekend driving.

“When people buy a more efficient car today, the next one they buy tends to be less efficient,” said David Rapson, lead author of the study.

The study examined California households with two vehicles from 2001 to 2007, using data from the Department of Motor Vehicles.

Researchers say pairing a high-fuel economy vehicle with a low-fuel economy vehicle reduces the household’s overall fuel economy.

“Both of these effects are large and undermine the goal of reducing fuel consumption. For people who care about reducing pollution from gasoline consumption, the results are a concern,” Rapson said.

Those with high-efficiency vehicles also tend to drive more which counteracts the fuel savings benefit of owning such a vehicle according to the study.

One reason given for having two different types of vehicles given is the trade-off in size and power. People drive the gas guzzlers on longer trips such as family vacations, or when more space is needed, but use the fuel-efficient vehicle for errands and city driving.

The study was funded by the California Air Resources Board. Researchers say they hope the new data will help shape policy decisions aimed at reducing carbon emissions.