Why you can't buy America's greenest car

Ask me or any auto expert what's the fastest car you can buy for any given amount, and we could easily cough up several options. Same for most luxurious, or off-roadable, or any other measurement. Yet there's one variation that's far harder to answer: What's the greenest, most environmentally friendly car you can buy today?

It's not easy knowing what's green in the auto industry. Last year's "Green Car of the Year" as chosen by the Green Car Journal — the Honda Accord hybrid and plug-in — doesn't even appear on the "Greenest Cars of 2014" list as chosen by the American Council for an Energy-Efficient Economy; only three of ACEEE's picks carry over to Kelly Blue Book's own eco-friendly list. Much of that's due to automakers trying to optimize their vehicles to solve different pollution problems;  high-efficiency diesels may score badly on smog ratings, while flex-fuel vehicles that reduce oil dependence by burning ethanol or natural gas get lower MPGs.

For most environmentally conscious car buyers, the first choice and last word in new cars is the Toyota Prius. And they're right; the Prius reliably gets 50 mpg in real-world driving, something no other midsize hybrid can claim at the moment. Yet even the Prius will, over the course of a typical year of travel, emit 3 tons of carbon dioxide, with another 0.7 tons of CO2 belched from the process used to make its gasoline.

Given that global warming poses the greatest environmental challenge of our age, reducing carbon emissions seems like the best simple yardstick for measuring a vehicle's environmental benefits. For those buyers who care, electric vehicles offer the best CO2-lowering alternative over gasoline. Yet electric vehicles still emit carbon dioxide — just from wherever the electricity they use is generated rather than a tailpipe. And because every utility uses different ratios of clean or carbon-burning energy, the results can vary widely.

By the EPA's own estimate, a Nissan Leaf charged in Washington, D.C., will save more than a ton of CO2 per year than a Prius — but in Chicago, the Prius actually coughs up less global warming gas annually than the Leaf. On a national average, the Leaf still burns 200 grams of CO2 per mile, saving less than half a ton of CO2 a year over the Prius.