Cars Vs. Climate: Eco-Driving Index Tells The Story

Consumers have gotten used to evaluating an automobile’s fuel performance based upon the U.S. Environmental Protection Agency’s fuel economy rating system. But what about a rating system that measures a car’s impact on the environment?

Researchers at the University of Michigan might have come up with just that. Their Eco-Driving Index estimates the average monthly amount of greenhouse gases produced by an individual U.S. driver who purchased a new vehicle that month.

EDI_February-2012

image via University of Michigan

The amount of greenhouse gasses emitted when using internal-combustion engines depends on the amount of fuel used. The EDI estimates the amount of fuel used (and thus the amount of greenhouse gasses emitted) by taking into account two primary variables: the fuel economy of the vehicle and the distance driven.

So according to this new index, how are we doing? Are greenhouse gasses increasing or decreasing? So far, the results look promising.

The EDI for February, which is the latest month for which data is available, stands at 0.81, compared to the baseline 1.0 in October 2007, the nominal start of the 2008 model year and the first for which the EPA started using the current fuel-economy rating system. That figure represents a 19 percent improvement over 2007. One year ago, (Feb 2011) the EDI stood at .85, which means that the index has decreased by .05 in one year’s time.

“The amount of greenhouse gases emitted when using internal-combustion engines depends on the amount of fuel used,” said Michael Sivak, research professor at the U-M Transportation Research Institute. “The EDI estimates the amount of fuel used (and thus the amount of greenhouse gases emitted) by taking into account two primary variables—the fuel economy of the vehicle and the distance driven.”

Sivak and his colleague Brandon Schoettle compute the monthly EDI by cross-multiplying the average amount of fuel used per distance driven by newly purchased vehicles (EDIf) and the distance driven per individual (EDId). The lower the value of the EDI, the smaller the environmental impact.

EDI, University of Michigan

image via Shutterstock

EDIf estimates the relative amount of fuel needed to drive a fixed distance. It is calculated as an inverse of the sales-weighted, average fuel economy of purchased new vehicles for each individual month. In turn, the average fuel economy (in mpg) is derived from the monthly sales figures of individual models and the EPA fuel-economy ratings for the respective models.

EDId provides information about the relative amount of driving per licensed driver. It starts with the estimates of the total distance driven in the United States each month as issued by the Federal Highway Administration. The researchers then adjust these raw distances to take into account the seasonal variations in driving, the varying number of days in a month, the continuously increasing number of drivers, and the so-called rebound effect (the increased amount of driving as a consequence of improved fuel economy of the new vehicle).

Steve Duda lives in West Seattle, WA with three dogs and a lot of outdoor gear. A part-time fly fishing fishing guide and full-time writer, Steve’s work has appeared in Rolling Stone, Seattle Weekly, American Angler, Fly Fish Journal, The Drake, Democracy Now! and many others.