But, I hear you say, a driver of an electric car in California will be getting cleaner power than a driver in the Midwest, which is heavily dependent on coal-fired power plants, right? The app lets you customize by region.

The paper, by Jessika E. Trancik, a professor of energy studies at the Massachusetts Institute of Technology, and colleagues, noted that the cars with the lowest emissions also tended to be the cheapest, which undercuts the notion many people have that living a greener life is necessarily more expensive. “Consumers can save money and save emissions at the same time,” Dr. Trancik said.

This app thrills me, because it turns out that I own the gas-powered car with the smallest carbon footprint — and physical footprint — on the market, according to the app: a Smart. I bought the tiny two-seater in 2008, and it has long been a point of contention between my wife and me. When people see us in the parking lot and ask how we like the car, I say “I love it!” and she says “I hate it” at the same time. She doesn’t like its bumpy ride or the finickiness of its transmission.