BOSTON – Pop the hood on the State of Massachusetts’ new plug-in hybrid and there is a regular Toyota Prius engine and a few dead leaves. But go for the spare tire in the trunk and you’ll get a look at the battery and bumper electrical outlet that make this car special, according to the Boston Globe.

So far, the State has been averaging 100 miles per gallon, according to David Cash, assistant secretary for policy at the State’s Executive Office of Energy and Environmental Affairs, which will be using the battery- and gas-powered car.

The white four-door — which was part of the state’s vehicle fleet and has been driven 46,558 miles — was retrofitted with a lithium-ion battery made by A123Systems of Watertown for about $10,000. It charges from a standard electrical outlet, and a dashboard monitor tracks its efficiency as it uses either battery or gas-power.

The plug-in will replace a 20-miles-per-gallon 2003 Ford Taurus. It is expected to save about 500 gallons of gas annually and reduce carbon dioxide emissions by about 4 tons. A regular gas-electric Prius gets about 50 miles per gallon in city driving.

The State has plans to retrofit and test-drive about 20 plug-in cars for official use — funding half the conversions with money from the federal Congestion Mitigation and Air Quality Improvement Program. The state also hopes to front half the cost, or $400,000, to purchase and retrofit another 20 cars to be tested by local companies, according to the Boston Globe.

And though the plug-ins will also aid the State’s effort to have 50 percent of its fleet made up of hybrids or other alternative-fuel vehicles by 2018.