ANN ARBOR, MI – A new study from the University of Michigan shows that greenhouse gas emissions per driver of a newly purchased vehicle are down 14 percent since 2007. Called the Eco-Driving Index (EDI), this record of emissions statistics was at 0.86 in April of 2011, compared to the baseline score of 1.0, established in October of 2007.

"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 UMTRI colleague Brandon Schoettle calculate the monthly EDI by cross-multiplying the average amount of fuel used by newly purchased vehicles (EDIf) when driving over a given distance and the distance driven by an individual (EDId). A lower EDI value means a smaller environmental impact.

The variable EDIf estimates the relative amount of fuel required to drive a set distance. Sivak and Schoettle calculate this value as an inverse of the sales-weighted fuel economy of new vehicles purchased in a given month. The duo calculates average the average fuel economy of purchased new vehicles based on the average sales figures for new models and the EPA fuel-economy ratings for those models.

The third variable, EDId, i.e the distance driven by an individual, uses estimates of the total distance driven in the U.S. on a monthly basis by individual drivers (reported by the Federal Highway Adminstration). The two researchers adjust these values based on the number of days in the month, the changing total number of drivers, seasonal differences, and take into account the tendency for drivers to travel more as vehicle fuel economy improves.