Banks slipped below 34%, down from 35% two years ago. The banks' leading auto lenders are JPMorgan Chase, Ally Financial, Wells Fargo, Capital One and Bank of America.

In many of these markets, consumers have a wealth of membership and loan options among credit unions. But credit union executives said their goal is not to poach from fellow credit unions, but to lure banking customers.

Golden 1 began expanding out of its base in Sacramento and the San Joaquin Valley in 2013, seeking borrowers north toward San Francisco and south toward Los Angeles. Both directions took it into other credit unions' territories, including two other credit unions among this group based in San Diego: California Coast and San Diego County.

“We really don't look to 'beat' the credit unions,” Greg Brown, Golden 1's chief lending officer, said. “I look at our competition to be some of the other financial institutions, like Wells Fargo, Chase, Bank of America, Cap One. There's plenty of business for credit unions across the state to take from the banks, because I believe we deliver a core value to the member.”

The group has had an out-sized impact on automotive loan growth. While the group remained in a range of 5.2% to 5.4% of total assets among U.S. credit unions, it has accounted for about a percentage point bump in growth each year. For example, automotive portfolio growth for the 12 months ending March 31 was 14.3% for the U.S. as a whole, but 13.3% when the 13 are excluded.

The group represents credit unions that in 2012 had a lower-than-average share of auto loans in their portfolio. Auto loans were 20% of total loans for the group, but 30% for rest of the nation's credit unions.

By 2015, the group of 13 had passed the rest of the nation, and at the end of March, 41% of the group's loans were in autos, compared with 34% for others.

On the other hand, the group came out of the recession with greater exposure to real estate. The 13 credit unions had 55% of their loans in first mortgages at the end of 2012, compared with 41% for the nation's other credit unions. By March 31, the gap had nearly closed, with the 13 having 42% of loans in first mortgages, compared with 41% for the rest of the nation.

Before the recession, Northwest Federal's portfolio was 26% auto loans and 62% real estate. At the end of March, the Washington-area credit union's portfolio was 50% auto loans and 40% real estate.

Harmonie Taddeo, vice president of marketing and communications, said Northwest Federal made a concerted effort to spread its rate risk.

“While mortgage lending is a core competency of the credit union and meets a critical need of our members as we partner on many levels with home loan solutions, we needed to diversify to also focus on shorter term loans,” she said.

Like many of the credit unions, Northwest Federal relied on CUSOs and other outside services to ramp up auto lending. It has partnerships with True Car and Carvana and offers a concierge auto-locating service to serve members based on where and how they want to buy.

Also like many others, part of Northwest Federal's success has been from indirect lending, which has allowed it to pick up new members in areas far from its branches, further diversifying its loans.

Golden 1 has built a network of about 1,200 auto dealerships, about 800 to 900 of them being frequent sources of indirect loans, which Brown said is the major force behind its growth.

“We've seen some decent growth in [direct lending], but it's tougher to grow a big portfolio based on that, no matter what institution you're in, whether you're a bank or credit union. The volume is at the dealerships,” he said.

Todd Lane, president/CEO of California Coast, wants to maintain a process so seamless that members never bump up against institutional boundaries between the credit union, its online venders and dealers. About half its members fund directly with the credit union, half at the dealerships.

“None of this is magic,” Lane said. “It's all stuff credit unions are doing.”