For snowboarders, surfers and mountain bikers, the car roof rack is an indispensable accessory, essential for weekend getaways.

Unfortunately, it’s also a bit of a gas hog.

A study published Monday found that a seemingly innocuous rack can cut mileage by as much as 25 percent, depending on its configuration and whether it’s fully loaded.

And while most cars on the road don’t have roof racks, the extra gasoline burned by those that do adds up. The study estimates that racks account for just under 1 percent of the nation’s fuel consumption for cars and light trucks, or roughly 100 million gallons of gas per year.

The study’s lead author, Alan Meier, has a long-standing interesting in the procedures used to test energy efficiency in equipment, appliances and cars. And the roof rack, he realized, isn’t included in mileage tests, even though it clearly makes cars less aerodynamic.

“I’m always interested in what gets counted and what doesn’t,” said Meier, a researcher with both the Lawrence Berkeley National Laboratory and the UC Davis Energy Efficiency Center.

“I was wondering, ‘Well, since racks aren’t counted, how much do they add overall?’” he said. “I found very little about that in the literature. And I found that it was a difficult question to answer.”

The effect on an individual car’s mileage varies based on the type of rack itself, whether it’s empty or loaded down with a wind-catching bicycle, as well as the type of car. For example, a highly aerodynamic car like a Prius will see a bigger mileage impact from a rack — even an empty one — than will a car whose shape isn’t quite so sleek.

“With a Prius, the moment you put on a rack, you’re going to greatly increase the aerodynamic resistance,” Meier said.

Getting a sense of how much gasoline the racks waste nationwide involved estimating how many racks are out there, how often they’re used versus left empty and how many miles their owners drive — both when the racks are in use and when they aren’t. Meier and fellow researcher Yuche Chen of the National Renewable Energy Laboratory used video surveys of highway traffic, as well as online forums and crowdsourcing, to come up with some of the estimates.

Simple changes, Meier said, could make a difference in the amount of fuel that roof racks waste.

Since most people who own roof racks only use them a small portion of the time, manufacturers could be encouraged to make the racks easier to remove when they’re not needed. They could also make the racks themselves more aerodynamic, with the efficiency of each design perhaps marked by a label.

“It seems to lend itself to an energy label, like we have with refrigerators and other appliances,” Meier said. “Consumers should be able to make a little better-informed decision.”

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com Twitter: @DavidBakerSF