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Exploring Fuel Efficiency: How Much Are You Spending on Gas?

By David Kroodsma

As the Memorial Day Weekend approaches, many of us are planning road trips — according to the American Automobile Association (AAA), over 30 million Americans will get in their personal vehicles for a weekend get-away, with many driving hundreds of miles.

Although gas prices have dropped slightly during the past few weeks, they are still a dollar per gallon higher than they were one year ago at this time. These prices have made many of us think hard about the efficiency of our vehicles — how much is it costing us to drive?

We have developed a tool to help you determine the costs of driving your car or truck, including how much carbon dioxide (CO2) your vehicle produces. Carbon dioxide is a key greenhouse gas that is contributing to global warming. Using the tool below, select your state, your car’s gas mileage, and how many miles you drive in a typical year (default values are set to national averages).

This widget — which you are welcome to embed on your webpage — also displays the CO2 emissions that result from a particular amount of driving. Gas price data are automatically updated daily from the AAA.


Don’t know how many miles you drive in a year?
If you don’t know, simply ask yourself if you drive more or less than the average American driver. The average passenger car in the U.S. travels about 10,000 miles per year, a bit less than two round-trips from New York City to Los Angeles. Adjust this number up or down depending on how much more or less you think you drive compared to average.

Don’t know your car’s gas mileage?
The average passenger car in the U.S. gets about 23 miles per gallon. If you don’t know your car’s gas mileage, you can estimate it using the U.S. EPA's online database.

Why do you save more gas by switching from a 20 mpg vehicle to one that gets 25 mpg compared to going from 25 mpg to 30 mpg?
According to our widget, if you drive 10,000 miles in a year and switch from a 20 mpg car to one that gets 25 mpg, you will save 100 gallons of gasoline per year. If instead you switch from a 25 mpg car to a 30 mpg car, you will only save 67 gallons. You save less gas with each mile per gallon of improvement.

In essence, the measure "miles per gallon" hides differences in gas use for cars with different fuel economies. In particular, it hides the diminishing returns with higher fuel economy cars. If, instead of using "miles per gallon," we used "gallons per 10,000 miles" (or maybe gallons per 100 miles) to measure fuel economy, the diminishing returns would be easy to see, as the above table illustrates. This is the way most countries measure a car’s efficiency. For example, most of Europe measures fuel economy as "liters of gasoline used per 100 km."

The amount of carbon dioxide your car produces depends only on how much gasoline your car uses.
Every gallon of gasoline your car burns creates about 19 pounds of CO2. It does not matter what type of car it is or how new the engine is — every gallon burned turns into exactly the same amount of CO2. But this CO2 represents only about three quarters of the total greenhouse gas emissions associated with a gallon of gasoline. Extracting crude oil from the ground, refining it into gasoline, and transporting the gasoline to the gas pump takes energy, and when emissions from these steps are added to emissions from burning the gas, the "lifecycle" greenhouse gas emissions amount to about 24 pounds of CO2 equivalent per gallon.

What is Carbon Dioxide Equivalent, or CO2-eq?
We say "carbon dioxide equivalent," written as CO2-eq, because other greenhouse gases, such as methane, are also released in gasoline’s "lifecycle" from crude oil in the ground to tailpipe emissions into the atmosphere. The global warming behavior of these gases is not exactly the same as for CO2, but it’s possible to estimate how much CO2 it would take to have the same effect. You can read about how CO2-eq is calculated on the EPA website, or at Skeptical Science.

The weight of greenhouse gases emitted from driving is surprising.
Driving a 23 mpg car for 10,000 miles results in about 10,500 pounds of CO2-eq added to the atmosphere. That’s about twice the weight of an average Hummer SUV or about four times the weight of a Toyota Corolla. It’s also the approximate weight of an adult African elephant. Since total greenhouse gas emissions per person in America averages about 50,000 pounds of CO2-eq, driving accounts for about one-fifth of an average American citizen’s greenhouse gas emissions.

How do 10,500 pounds of carbon dioxide compare to annual emissions from other activities?
Estimated carbon emissions per year for a number of everyday activities are shown on the graph below. To see the assumptions behind each estimate, click on different portions of the graph.  

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