About a third of all recalled cars and trucks don’t get repaired, and about one out of every seven vehicles, or 36 million, still on the road have an unrepaired defect, according to data compiled by regulators, safety advocates and the CarFax vehicle history company.

That means recalled cars from automakers as varied as General Motors Co., Honda Motor Co., Chrysler Group LLC and Toyota Motor Corp. will remain unrepaired on the roads, in some cases putting drivers at risk. GM Chief Executive Officer Mary Barra will struggle to achieve her stated goal of repairing 100 percent of the 2.19 million small cars recalled for an ignition fault linked to 13 deaths.

“You’ve got 15 percent to 20 percent right off the bat who are probably never going to come in,” said John McEleney, a Chevrolet and Toyota dealer in Clinton, Iowa. “They never get these corrections made for whatever reason, as much as the manufacturers and the dealers try.”

This year, total recalls already have exceeded 12 million vehicles — more than half of last year’s 22 million — according to government data.

To be sure, the publicity surrounding GM’s recall may increase the likelihood of customers bringing in their cars, said George Hoffer, a professor who has studied recall response rates at the University of Richmond in Virginia. In the high- profile case of Toyota’s recall of cars that accelerated unexpectedly, the rate of repair for cars with sticky gas pedals reached 88 percent, according to government data.

Warranty Coverage

Drivers are most likely to get their cars fixed if they’re still covered by a warranty because dealers typically fix or replace defective parts when owners bring in their vehicles for regular service. Dealers will always fix a recall because they get reimbursed by the car companies. After the warranty runs out or the car is sold, however, all bets are off — even in cases where there’s a recall notice.

Sometimes drivers just can’t be bothered, especially if they’ve been driving the car for years without incident, said Ray Chapman, 64, who owns a 2006 Chevy HHR that has been recalled. He said he bought the black wagon in the fall of 2006 and since then has put 170,000 miles on it commuting to work.

“I may not be the first in line, but I’m well aware of the potential problem,” said Chapman, an assistant programmer from West Suffield, Connecticut. “But I’m the kind of person who doesn’t get scared by the headlines. I can assess the real risk and I’ve driven a lot of miles with no issues. I don’t expect it to happen in the next month.”

Dealer Repairs

Some drivers fret that the dealer will try and sell them other services, Hoffer said.

“People who bought it used don’t have a relationship with the dealer, and they’re afraid that when they take it in the dealer is going to mine their car for other things to fix,” he said. “People have a tendency to let sleeping dogs lie.”

In many cases, drivers simply don’t know the model they’re driving has been recalled. Perhaps they’ve moved and don’t receive a recall letter from the automaker. If they’re the second or third owner, they won’t even be sent a notice.

Collapsed Suspension

Donna Voag’s 20-year-old son, Christopher, was driving his 2001 Hyundai Elantra to the gas station from their home in Bethlehem, Pennsylvania, late one evening last August when the front suspension collapsed. He had no idea the vehicle had been recalled four years earlier for that defect, she said.

“Thank God he was going slow and decided to go to the gas station first, or otherwise he’d have been on the highway,” said Voag, 54, who manages a family shoe store. “He was very upset when he came home. He said. ‘Something really bad happened to my car.’ It was very dangerous.”

As cars get older, people are more likely to use a repair shop that doesn’t get reimbursed for recall repairs or do the work themselves, particularly on cheaper models, Hoffer said.

In 2009, Ford Motor Co. recalled 4.5 million vehicles ranging from minivans to SUVs to pickup trucks for a part that could spark a blaze even if the vehicles were sitting in the driveway. Because the models stretched back to 1992, Ford was unable to reach 600,000 owners, according to National Highway Traffic Safety Administration records.

Automakers are required to submit quarterly reports of recall repair progress to NHTSA for 18 months after the recall is announced. In a 2011 study of the recall system, the U.S. Government Accountability Office reported that after 18 months, the completion rate was about 65 percent. In 2011, Hyundai reported that 39 percent of 160,904 vehicles recalled for suspension problems had been repaired. The Korean automaker said another 6,852 owners couldn’t be reached.

Agency Rules

NHTSA recently instituted new rules to make the notices stand out more to owners, said Stephen Selander, an attorney in Rochester, Michigan, who used to work at GM and now advises automakers on recall issues.

“If NHTSA decides that the rate is too low, on a particular recall, because the recall is significant or it’s just a really low rate, they can force the manufacturer to issue a second notice to try and get more people in,” he said.

In 2012, Chrysler began e-mailing, phoning and mailing recall notices to vehicle owners in an effort to boost repair rates. The average response has risen to 80 percent after 18 months from 70 percent before the program, spokesman Eric Mayne said in an e-mailed statement.

Cobalts, Ions

Because many of GM’s recalled models were inexpensive, small cars like the Chevrolet Cobalt and Saturn Ion that have been on the road close to a decade, the automaker will struggle to fix a high percentage of them, Hoffer said.

Barra vowed earlier this month to deploy a “special team that’s going to be dedicated to making sure we contact each customer and make arrangements for them to come in.” GM may even try to reach customers with OnStar, the communication system that links GM vehicles to live operators for directions, information and emergency calls.

Ignition switch repairs began on April 11, Kevin Kelly, a GM spokesman, said yesterday in an e-mailed statement. More than 22,000 customers have requested loaners while awaiting repairs, he said. As of yesterday, McEleney’s Chevrolet dealership had repaired 10 vehicles.

–With assistance from Tim Higgins and Keith Naughton in Detroit, Mark Clothier in Southfield, Michigan and Alan Ohnsman in Los Angeles.