The idea that gasoline cars might cause less environmental harm than electric vehicles seems impossibly backwards. But consider the following thought experiment before you dismiss it out of hand.

A view from the tailpipe gives EVs a clear edge: no emissions, no pollution, no problem. Shift the view to that of a smokestack, though, and we get a much different picture. The EV that caused no environmental damage on the road during the day still needs to be charged at night. This requires a great deal of electricity generated by a power plant somewhere, and if that power plant runs on coal, it’s not hard to imagine it spewing more emissions from a smokestack than a comparable gas car coughed up from a tailpipe.

So the truth of the matter hinges on perspective—and, it turns out, geography. That’s the sobering lesson from an incredibly sophisticated new working study by a group of economists. Using a fine-grained, county-level measure of U.S. vehicle emissions traced to tailpipes and electricity grids, the researchers mapped where gas cars and EVs cause more respective pollution. In some places electrics do so much relative harm that instead of being subsidized, as is currently the case, they should actually be taxed.

“What we find is that the benefits are substantially different depending on where you are in the country,” study co-author Stephen Holland of the University of North Carolina, Greensboro, tells CityLab. “The real big take-home message is: location, location, location.”