Automakers continue to improve the average fuel economy ratings of vehicles, including full-size pickups like the Ram 1500 EcoDiesel.

While some might think that rising sales of new vehicles would eliminate the incentive for automakers to continue improving the fuel economy of cars and trucks, the ratings of brand new vehicles driven out of showrooms across the United States has continued to increase since the start of the 2014 model year, according to University of Michigan.

The University of Michigan the University of Michigan Transportation Research Institute (UMTRI) reported the average fuel economy rating of new vehicles sold in the U.S. was at a record-high 25.2 mpg in February.

It was the fifth straight month the fuel-economy figure has topped 25 mpg, reflecting, in part, the tougher standards mandated by the U.S. Environmental Protection Agency, according to U-M researchers.

Average fuel economy or the window-sticker values of cars, light trucks, vans and SUVs purchased last month was up 2.5 mpg from just three years ago and up 5.1 mpg from October 2007, the first full month of monitoring, according to UMTRI researchers Michael Sivak and Brandon Schoettle.

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

For the second consecutive month, the EDI registered its best mark ever: 0.78 (the lower the value, the better) during December. The index currently shows that emissions of greenhouse gases per driver of newly purchased vehicles are down 22%, overall, since October 2007.

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The average sales-weighted fuel economy was calculated from the monthly sales of individual models of light-duty vehicles (cars, SUVs, vans and pickup trucks) and the combined city/highway fuel-economy ratings published in the EPA Fuel Economy Guide. The fuel-economy information was available for 99.8% of vehicles purchased.

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The EDI is a national index that estimates the average monthly amount of greenhouse gases produced by an individual U.S. driver who has purchased a new light-duty vehicle that month.

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The amount of greenhouse gases 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 gases emitted – by taking into account the fuel economy of the vehicle and the distances driven.