In Washington state, the best all-electric cars cause so little carbon pollution that it’s like getting 383 miles per gallon of fossil fuel, even if you take into account the energy it took to make the battery.

This is because three-fourths of the state’s electric supply is generated by river dams.

But in many other states, a gasoline-only car that gets 34 to 37 mpg would be cleaner than an all-electric car, because the power supply comes mainly from burning coal.

This is according to a report released Thursday by Princeton, N.J.-based Climate Central, a nonprofit group focused on climate change.

The researchers addressed two common questions about electric cars: First, would the environmental damage of generating the electricity make a plug-in car dirtier than a car that uses gasoline? And second, would the carbon emitted in making the battery cancel the carbon savings from driving a plug-in car?

“An electric car is only as good for the climate as the electricity used to power it,” the report begins. Electricity produced from fossil fuels causes more carbon emissions than that from hydro or nuclear plants.

The advantage in Washington state is second only to Vermont, which uses predominantly nuclear power, supplemented by hydro and other renewable energy sources. Idaho, Oregon and California are similar to Washington.

Kentucky, Indiana, West Virginia, Wyoming and New Mexico rank as the dirtiest states for power generation. Those are the places where an all-gasoline car can be cleaner than all electric. South Dakota ranks fifth for its clean hydropower, while oil-rich North Dakota has one of the dirtiest electric grids, ranked 42nd.

Even in hydro-powered states, there’s no free lunch.

“One of the surprising things to us is how much the emissions to make the battery matter,” said Alyson Kenward, a senior scientist and co-author. Battery manufacturing emits 10,000 to 40,000 pounds of carbon per battery and electrical components, the report says.

This means, for instance, that in Washington state you would have to drive a plug-in Honda Fit 20,430 miles to overcome the initial “carbon debt,” compared to driving a gasoline-hybrid Toyota Prius, she said.

There are 37 states reported where a gasoline-hybrid Toyota Prius is more benign than an all electric Nissan Leaf, even after 100,000 miles on the road.

However, the report doesn’t capture all the ecological burdens, including the radioactive waste left by nuclear power or the historic destruction of salmon runs by Northwest hydropower.

Hybrid cars pair a gasoline engine with a battery that constantly recharges from the kinetic energy of the moving car. The hybrid batteries require less energy to make than an all-electric battery. A Chevy Volt runs for about 40 miles off a plug-in battery and carries a gasoline engine to extend its range. (General Motors this week reduced the sticker price by $5,000 to about $35,000, not including another $7,000 or so in possible government rebates, CNN reported.)

Washington, Oregon and California are developing the West Coast Green Highway, with charging stations provided every 25 to 50 miles, allowing all-electric driving.

The cleanest cars for Washington state were the electric Honda Fit, the Nissan Leaf, a plug-in Toyota Prius, Ford Focus Electric and Ford C-Max Energi.

Scientists have identified carbon emissions as a contributor to global warming. In Washington state, transportation is the leading source of carbon pollution.