Low fuel prices are encouraging consumers to buy more full-size pickups and utilities than cars, driving down fleet efficiency.

After years of progress, the drive to improve the efficiency numbers of the U.S. automobile fleet has stalled as consumers opt for light-duty trucks and utility vehicles rather than passenger cars in the current sales frenzy.

While domestic, Asian and European carmakers are enjoying some of their best sales ever in the U.S., the sales of passenger cars to American customers has actually fallen during the first 11 months of 2015.

The numbers are stark. Sales of cars of all types have dropped 2.2%, while sales of trucks and utility vehicles have increased 12.7%. The shift means that trucks will outsell cars in the U.S. by roughly 1 million units this year, according to numbers collected by Auto Data. Last year the split was roughly even with a small tilt towards the truck side of the business.

The University of Michigan in its latest monthly estimate of new-car purchases estimates that the fuel economy of new vehicles sold in the U.S. dropped for the fourth straight month and to its lowest point this year.

Average fuel economy (window-sticker value) of cars, light trucks, vans and SUVs purchased in November was 25 mpg, down 0.1 mpg from the revised value for October, noted the report by U-M’s Transportation Research Institute.

(Fuel prices could be in for even sharper decline. For more, Click Here.)

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

Overall, fuel economy is down 0.8 mpg from the peak reached in August 2014, but up 4.9 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.

During September, the EDI rose to 0.83 (the lower the value, the better) from 0.81 in August. The index currently shows emissions of greenhouse gases per driver of newly purchased vehicles are down 17%, overall, since October 2007. EDI reached its best level (0.78) in August 2014, UMTRI reported.

(Click Here to see how November truck sales set industry up for all-time record.)

The shift towards trucks appears to reflect the growing confidence among consumers that fuel prices will remain stable and are unlikely to surge upwards in the foreseeable future.

AAA reported this week that the average price of a gallon of gasoline in the United States had dropped to $2.01 per gallon, its lowest point since the Great Recession of 2009.

The relatively low price of crude oil is helping to sustain year-over-year discounts in the price of gasoline nationwide. Retail averages are down more than 50 cents per gallon in 48 states and Washington, D.C., and motorists in a dozen states are saving at least 75 cents per gallon versus this same date last year.

OPEC’s decision at its meeting last week to sustain its current production levels sent ripples through the global oil market and has contributed to lower oil prices. The cartel is not scheduled to reconvene until June 2016, and in the interim the imbalance between supply and demand will likely persist, analysts suggest.

(To see more about gas prices tumbling in November, Click Here.)

AAA said the domestic oil market is also reflecting signs of oversupply. Both crude oil and gasoline inventories are approaching record levels even as lower prices impact exploration and production. The latest data from the Bureau of Labor Statistics, the domestic oil and gas extraction sector lost upwards of 2,400 jobs during the month of November, AAA noted. This follows the October report which showed the sector shedding approximately 2,700 jobs.