These days, many individuals are trying to come up with ways to reduce their “carbon footprint.” How much does your car matter to your “carbon footprint?” Maybe we should be asking, “What is your car’s “carbon tire track?”

About 29% of the 6 billion tons of carbon dioxide Americans send into the atmosphere every year comes from transportation. About two thirds of U.S. transportation emissions are by passenger vehicles, if you include light and medium-duty trucks (EPA.gov). So yes, transportation is an important part of the problem.

To assess the carbon output of your car, all you need is a few numbers. How many miles to the gallon do you get? How many miles do you drive per year? Say you get 20 miles per gallon and you drive 15,000 miles per year, as an average Mainer. You divide the miles you drive by the mpg and get 750, the number of gallons of gas you use in a year.

So every gallon of gas burned produces 19.6 pounds of carbon dioxide, believe it or not! Multiplying 750 by 19.6 pounds you get 14,700 pounds (7.6 tons) of CO2 per year that you are adding to the atmosphere just from driving a car getting 20 mpg.

If you have a car that gets 40 mpg, obviously you are burning half as much gas – producing around 3.8 tons of CO2. Since the average American is responsible for 18 tons of carbon dioxide emissions per year, your choice of a vehicle and its “carbon tire track” can make an impressive difference in your overall carbon footprint. In this example, switching from 20 to 40 mpg will likely reduce your total CO2 output by 20%.

Electric cars do still have a “carbon tire track,” since they use plenty of energy – it’s just in the form of electricity. Electricity involves less waste and no by-products in using it as an energy source, so it is generally a good idea in terms of emissions. But the electricity that most of us buy is generated by a variety of sources, including fossil fuels. In Maine, we have a fairly high contribution of renewables like hydro and wind, but the CO2 is not nothing.

There is a cool website called fueleconomy.gov operated by the U.S. Department of Energy that allows you to see the estimated greenhouse gas emissions associated with driving any model or year of electric vehicle, specially targeted to your zip code’s electricity sources. For instance, it tells us that if we were driving a 2019 Chevy Bolt our CO2 emissions rate would be 80 grams per mile. That would translate, in our 15,000 miles of annual driving, to 2,600 pounds of CO2, or 1.3 tons, about a third of what our 40mpg Honda Fit produces.

There are other issues involved in looking at the total footprint of electric cars because of their batteries and other manufacturing issues, but CO2 emissions are definitely minimized.

In future columns we will examine other areas of potential impact for average individuals trying to do practical things to limit their contribution to our greenhouse gas emissions. There are many things people do, hoping to make a difference – but you might be surprised at the comparisons among various measures when you take a quantitative approach using established values for emissions.

Meanwhile, we hope this analysis of “carbon tire track” is useful to you in planning your next vehicle purchase!

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side. He can be reached at pauls@maine.edu. Cynthia Stancioff, MA, Public Administration, is an amateur naturalist and wordsmith.

NOTE: The authors wish to correct an error in their previous column on heat pumps: It should have read “When the temperatures outside are below a certain level another automatic source of heat can supplement the heat pump as needed.”