As sales of full-size trucks, like the F-150, continue to tick upwards, fuel efficiency averages for new vehicles slides downward.

As the sales of trucks and utility vehicles continue to surge, the fuel-economy of new vehicles sold in the U.S. continues to drop, according to a monthly study by the University of Michigan.

In fact researchers at the University of Michigan Transportation Research Institute found the fuel economy of new vehicles sold in the U.S. as rated by the federal Environmental Protection Agency fell for the second month in a row. The average fuel economy, or window-sticker values, of cars, light trucks, vans and SUVs purchased in September was 25.2 mpg, down 0.1 mpg from August.

“This decline likely reflects the decreased price of gasoline in September, and the consequent increased sales of pickup trucks, SUVs and crossovers,” said Michael Sivak, a research professor at UMTRI.

Overall, fuel economy is down 0.6 mpg from the peak reached in August 2014, but up 5.1 mpg from October 2007—the first full month of monitoring by Sivak and colleague Brandon Schoettle.

In addition to average fuel economy, Sivak and Schoettle issued a monthly update of their national Eco-Driving Index, which estimates the average monthly emissions generated by an individual U.S. driver, Sivak said.

Gas prices continue sliding, encouraging folks to buy less fuel efficient vehicles.

During July, the EDI remained at 0.82 (the lower the value, the better) for the third straight month. The index currently shows emissions of greenhouse gases per driver of newly purchased vehicles are down 18 percent, overall, since October 2007. EDI reached its best level (0.78) in August 2014.

Meanwhile, gasoline prices remain relatively stable, with AAA reporting the national average price for regular unleaded gasoline holding at $2.29 per gallon this week. The average price is up by fractions of a penny compared to one week ago, but has moved lower for 43 of the past 49 days, providing drivers with a savings of 12 cents per gallon month-over-month, AAA reported.

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Significant yearly discounts remain with drivers saving $1.01 per gallon on average, largely attributed to cheaper crude oil and ample gasoline supplies, AAA said.

AAA also noted that retail averages have declined 51 cents per gallon since reaching the 2015 peak price of $2.80 per gallon on June 15. The national average tends to move lower during the autumn and winter months due to seasonal declines in both driving and gasoline demand, and pump prices have fallen during the month of October for three years in a row.

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Despite refineries going offline to conduct scheduled maintenance, there should still be more than enough gasoline to meet demand because people tend to drive less this time of year. Barring any major disruptions in supply, the national average is expected to move lower by the end of the year, and for the first time since 2009, could fall below the $2 per gallon benchmark.

Drivers in every state are paying an average price at the pump below $3 per gallon for the first time since 2009. Averages on the West Coast remain among some of the highest in the nation, with California at $2.94 unseating Alaska at $2.91 as the nation’s most expensive market for retail gasoline.

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The global supply of oil continues to exceed demand on global energy markets. Saudi Arabia, the world’s largest exporter of petroleum and the swing member of the Organization of Petroleum Exporting Countries, has announced plans to further cut prices to Asia in an effort to protect its market share. Russia is reportedly ready to enter talks with OPEC and other producing countries about possible reductions in output.