In separate reports, dealership buy-sell advisers Erin Kerrigan and Alan Haig predict their business will pick up in 2018, after two years of fewer transactions, because of the benefits from federal tax-law changes, dealers' desire to grow in scale and stable dealership profits.

"We see more public groups getting back into the acquisition game," said Kerrigan, managing director of Kerrigan Advisors in Irvine, Calif. "We continue to have family offices and high-net-worth investors looking to back growing dealership groups, so there's capital for those dealers who want to grow."

Many buyers and sellers believe the only way to survive as the industry embraces electric and selfdriving vehicles, ride-hailing and subscription car services, is to grow. If a dealer cannot expand fast enough, that dealer may choose to sell while prices are still healthy, Kerrigan said. Meanwhile, "There is a sense of urgency for some buyers to become $1 billion-plus dealership groups," she said.