The Massachusetts Bay Transportation Authority could save $250 million over six years by bringing the cost of maintaining its bus fleet in line with other transit agencies around the country, according to a report released Wednesday by the Pioneer Institute.

Citing figures from the U.S. Department of Transportation's transit database, the study found the MBTA's average bus maintenance cost of $3.76 per mile was the fourth highest out of 379 transit agencies, and nearly twice the average of systems that operate at least 100 buses.

The MBTA, in responding to the report, cited the ever-increasing costs of maintaining aging vehicles and said it had made significant strides in reducing personnel costs in recent years.

The report from Pioneer, a conservative-leaning think tank, comes as lawmakers grapple with a transportation finance bill that calls for about $500 million in new taxes and seeks to plug a $118 million deficit facing the T in the fiscal year starting July 1.

According to the institute, the MBTA had the second highest number of full-time maintenance employees per total number of bus miles traveled. The average compensation for a full-time employee, including overtime and benefits, was $111,634 in 2012.

"Bus maintenance garages are overstaffed, employees are overpaid and the performance is way below efficiency," said Gregory Sullivan, Pioneer's research director and a former state inspector general.

Sullivan, however, did not pin the bulk of the blame on MBTA management, instead pointing the finger at a 20-year-old anti-privatization law that sharply restricts the ability of state agencies to contract out services.

The law requires private contractors to pay employees the same wages and health care benefits as they would receive in the public sector, and promise savings higher than what would be achieved if a public agency was operating at maximum efficiency.

The law has effectively prevented the T from competitive purchasing, Sullivan said.

"We have no doubt the MBTA is trying to do the right thing," he said. "But they are handcuffed. They are handcuffed by a culture at MBTA garages that says `don't kill the jobs."'

The report recommends that the state exempt the transit system from the requirements of the law and give the MBTA more flexibility to seek private contracts for bus maintenance.

Reducing maintenance costs to the average of those at comparable transit systems would save $250 million over five years, the report said.

The MBTA disputed some of the findings in the study, insisting it was doing "more with less in order to maintain our high standards for customer service and passenger safety."

The agency said the costs of maintaining aging vehicles and infrastructure were expected to grow at a rate nearly four times that of wages and benefits over the next five years.

The $34.88 hourly wage for bus machinists were in line with several other large transit systems including San Francisco, Chicago and New York, the T said, adding that it had reduced its total workforce by more than 400 over the last five years and cut total overtime by nearly $3 million in the last fiscal year.