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Gas prices have lots of us thinking about every drop of fuel that goes into our cars. But what about every puff of carbon dioxide that comes out?

That’s not something we dwell on much. That’s because a vehicle’s CO₂ emissions aren’t easy to quantify while sitting behind the wheel—much less visualize, since the gas comes out the tailpipe and is invisible anyway.

Sitting in front of a computer with Internet access, it’s a different matter. That’s because the government’s done almost all the work for us.

Overall, each and every American is responsible for emitting 20.15 metric tons of carbon dioxide a year, according to 2005 Department of Energy statistics. Transportation—think planes, trains, automobiles and the like—accounted for about one-third of that per capita total. The average passenger vehicle alone is responsible for 5.2 metric tons of carbon dioxide emissions a year, according to the Environmental Protection Agency (EPA).

It’s easy to calculate a vehicle’s carbon dioxide emissions, though I wouldn’t recommend trying it while driving. First off, the EPA tells us that a gallon of gasoline produces 8.8 kilograms, or 19.4 pounds, of CO₂ when combusted. And that same gallon carries the average American passenger vehicle an EPA-estimated 20.3 miles, though your own set of wheels may have better or worse fuel economy.

Since the EPA also estimates that the average vehicle travels 12,000 miles a year, it’s a snap to calculate how much gasoline it burns to do so:  about 591 gallons (that’s 12,000 miles traveled in a year ÷ 20.3 miles per gallon = 591 gallons of gasoline burned).