ANN ARBOR — After recently topping 24 miles per gallon for the first time ever, fuel economy of all new vehicles sold in the United States slipped back below that mark last month, 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 April was 23.9 mpg, down from 24.1 in March, but the same as in February. Despite the drop, fuel economy is up 3.8 mpg (or 19 percent) from October 2007, the first month of monitoring by UMTRI researchers Michael Sivak and Brandon Schoettle.

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“The decline likely reflects the slight reduction in the price of gasoline during the second half of April,” Sivak said.

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.

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During February, the EDI stood at 0.81, an improvement from 0.83 in January and 0.86 in December (the lower the value, the better). The index currently shows that emissions of greenhouse gases per driver of newly purchased vehicles are down 19 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.

For April, unadjusted CAFE performance was 29.3 mpg, down from 29.6 mpg in March, but an increase of 19 percent (4.6 mpg) since October 2007.

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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