Expanding on a growing list of environmental initiatives in San Francisco, Mayor Gavin Newsom called on leaders of Bay Area cities and counties Thursday to join him in urging automakers to produce plug-in hybrid cars.

At a press conference at a southeast San Francisco auto repair shop that converts standard hybrids to plug-ins, Newsom said he would mail letters to Bay Area mayors and other leaders asking them to join him in an offer to buy the autos if the industry will make them.

"This is an open purchase order where we will commit to purchasing these low-emission cars and we will use them to replace the vehicles in our existing fleet," Newsom said. "We want to create our own market for these cars."

Newsom stood in front of three hybrid plugs-ins that Pat’s Garage converted for the city. He said he would also push his idea at the U.S. Conference of Mayors in June.

A plug-in is an ordinary hybrid with an electric motor and gasoline engine that has been modified - usually by adding a lithium ion battery to the existing standard car battery - so it can go a lot farther on electric power than it normally does.

To recharge the cars, drivers need only plug into a standard outlet. The cost for a total recharge is estimated at less than $1.

In July, a study by the National Resources Defense Council said greenhouse gas emissions and domestic oil consumption would drop sharply if plug-in hybrid technology became widespread by 2050.

Mass production of the vehicles appears to be years away, however, because of concerns over the lithium batteries catching fire and wearing out sooner than the car itself is supposed to.

Officials at Toyota, which produces the popular Prius hybrid car, have said the company is studying the possibility of adding plug-in hybrids to its models, as are other major carmakers.

Pat Cadam, owner of Pat’s Garage at 26th and Indiana streets, has expanded his auto shop to include a new hybrid conversion venture called Green Gears. Cadam said he has transformed about 35 hybrids to plug-ins and believes he’ll modify many more before the big automakers come around.

For about $20,000, Cadam’s mechanics can plunk a 145-pound lithium battery in the trunk of a Prius and it will get up to 150 miles per gallon of gas, he said.

"Converting these cars is not a backyard project; these are high-voltage batteries," Cadman said.

In recent months, Newsom and the San Francisco Board of Supervisors have promoted a number of environmental initiatives.

In December, Newsom announced a plan for a carbon offset fund for city departments. That followed strict green building standards for new construction and a conversion of the city’s fleet of diesel buses and vehicles to biodiesel.

In 2007, the Board of Supervisors banned the use of plastic bags at large grocery stores in the city. Earlier this month, the board gave initial approval of a measure requiring taxi companies to convert their fleets to low-emission vehicles by 2012.