InsuranceQuotes.com says a 20-year-old single woman will pay 28 percent more than a woman the same age who is married.
"It's based on the assumption that individuals—particularly young individuals—get more serious about being better drivers when they're married," said Bob Hunter, former commissioner at the Texas Department of Insurance and current director of insurance at the Washington-based Consumer Federation of America, a consumer advocacy organization. "It's questionable, but insurers maintain that claims data back it up."
The study shows the gap in auto insurance premiums for married and unmarried drivers narrows as people age—that is, until they turn 60.
The InsuranceQuotes.com study found monthly premiums stabilize around age 30, and then steadily decrease for men and woman until they turn 60. After that, they start to edge higher.
Read MoreDriving violations drive up insurance, study shows
Even then, a 60-year-old single male driver is paying, on average, 60 percent less for auto insurance than a 20-year-old single man. For women, the difference is an average of 50 percent less.
As always, how many tickets a person collects—and certainly, how many accidents and claims they file—will push their premiums higher regardless of age, gender or marital status.
—By CNBC's Phil LeBeau.
Questions? Comments? BehindTheWheel@cnbc.com.