ANN ARBOR — For the second straight month, fuel economy of all new vehicles sold in the United States fell by 0.2 mpg, likely reflecting a slight drop in gas prices, say researchers at the University of Michigan Transportation Research Institute.

Average fuel economy (window-sticker values) of cars, light trucks, minivans and SUVs purchased in May was 23.7 mpg, down from 23.9 in April and 24.1 in March, but still the fourth best month on record. It’s also up 3.6 mpg or 18 percent from October 2007, the first month of monitoring by UMTRI researchers Michael Sivak and Brandon Schoettle.

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In addition to average fuel economy, Sivak and Schoettle issued their monthly update of their national Eco-Driving Index, which estimates the average monthly emissions generated by an individual U.S. driver. The EDI takes into account both vehicle fuel economy and distance driven, the latter relying on data that are published with a two-month lag.

During March, the EDI stood at 0.83, worse than the 0.81 mark in February, but the same as in January (the lower the value, the better). The index currently shows that emissions of greenhouse gases per driver of newly purchased vehicles are down 17 percent, overall, since October 2007.

Finally, Sivak and Schoettle report the unadjusted Corporate Average Fuel Economy performance. This index is based on a different set of EPA ratings than window-sticker values.

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For May, unadjusted CAFE performance was 29.1 mpg, down from 29.3 in April and 29.6 mpg in March, but an increase of 18 percent (4.4 mpg) since October 2007.

Fuel economy calculations, along with a graph and table of current and recent mpg, are at

Eco-Driving Index calculations, along with a graph and table of current and recent values, are at

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Unadjusted CAFE performance, along with a graph and table of current and recent mpg, are at