A Tailpipe Dream

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).

From there, it’s just as easy to calculate CO₂ emissions:  5,201 kilograms or 5.2 metric tons per vehicle (that is, 591 gallons of gasoline burned in a year × 8.8 kilograms of CO₂ produced per gallon = 5,201 kilograms CO₂ emitted).

I freely mixed English and metric units in the above calculations but that’s because gasoline’s typically sold by the gallon and CO₂ emissions are counted up by the metric ton. If we stick to just English units, a good rule of thumb is that driving produces slightly less than a pound of carbon dioxide per mile traveled. That’s assuming 20.3 miles per gallon. If your vehicle gets 10 MPG, then it’s roughly two pounds a mile. If it gets 40 MPG, then it’s about a half-pound.

Getting that information to drivers in the United States could get them thinking about the real-world effect their vehicles have on CO₂ emissions, the atmosphere and, ultimately, climate change.  It’s information that would fit neatly on the window labels that now list a new car’s fuel economy.

That information is already is on labels in  Europe, where they must display both a new car’s fuel economy and its carbon dioxide emissions. The latter, expressed in grams of CO₂ per kilometer traveled, also is used as a regulatory benchmark. The European Commission recently proposed requiring automakers to drop their average CO₂ emissions to 120 grams/kilometer by 2012. That’s 0.42 pounds a mile.

Maybe the U.S. could require a similar amount of disclosure, but do Europe one better, too. It could further require the instrument panels of our cars, trucks and sport utility vehicles to include a gauge that displays a running average of the vehicle’s carbon dioxide emissions, as expressed in pounds per mile traveled. Such an instrument, call it the “carbonometer,” would give drivers a clear view of the CO₂ coming out the tailpipe—even if it is invisible.