TORONTO -- Ontario drivers may have overpaid billions of dollars in insurance because a profit benchmark for the industry is set way too high by Ontario's regulator, a new study has found.

Consumers likely overpaid by $3 to $4 billion between 2001 and 2013 -- $840 million in 2013 alone -- according to the Schulich School of Business study commissioned by the Ontario Trial Lawyers Association.

The study pointed to the profit benchmark set by the Financial Services Commission of Ontario at 11 per cent return on equity as being too high.

"Consequently, premiums have been too high, and consumers in Ontario have been paying too much for auto insurance," the study authors wrote.

"There is significant room to reduce rates. The combination of a return on equity cap of 5.8 per cent, the 10-year rolling average for 2013, and a lower operating cost assumption could reduce auto insurance premiums by at least 7.9 per cent based on 2013 data."

The cap should be no more than about 5.5 per cent, given low interest rates, the study authors concluded.

The Ontario trial lawyers called on the government to reduce the profit cap and to have the auditor general investigate auto insurance in Ontario.

The Liberal government has promised to cut auto insurance rates by an average of 15 per cent from where they were in the summer of 2013 by this August.

A spokeswoman for Finance Minister Charles Sousa said that auto insurance rates have declined by more than six per cent on average since August 2013, but his press secretary said they "look forward to reviewing the report."

"Our government is always looking for ways to continue to reduce rates and fight fraud in the auto insurance system," Kelsey Ingram wrote in a statement. But she said reviewing private companies would not be within the auditor general's mandate.

Ontario's opposition parties are skeptical that drivers will see that 15-per-cent cut by this summer.

"The target is only a couple of months away...so they are poised to fail miserably," said Progressive Conservative critic Vic Fedeli.

NDP critic Jagmeet Singh said changes in 2010 to the regulatory system resulted in cost savings for the insurance companies. This study bolsters his belief that the insurance industry is making a "significant profit," he said.

"There is absolutely enough profit in the system as it stands for the government to make that reduction with the existing savings that the insurance industry is already enjoying," Singh said.