As carmakers work to meet tightening global emissions standards, new-car average fuel economy has steadily improved over the past several years.
At least, until last month.
In November, the sales-weighted average fuel economy of new light vehicles sold in the U.S. fell for the first time, according to the Wards Auto Fuel Economy Index.
That year-over-year decline wasn't enormous, admittedly. The index showed average fuel economy at 24.8 mpg, a decrease of 0.1 percent over the same period in 2013.
November also saw a significant decrease in fuel prices, which Wards says enticed consumers back towards less-efficient vehicles.
The national average gasoline price last month was $2.99 per gallon. That represents a 9.8-percent decrease compared to November 2013, and the first time the average price has dropped below $3 per gallon since November 2010.
The average price of diesel was also down, to $3.64 a gallon. That's a decrease of 5.0 percent over the same month last year, and the lowest average price since February 2011.
The index also recorded a shift in market share from cars to light trucks--with the latter increasing its share 53.0 percent at this time in 2013 to 55.4 percent--but with increases in average fuel economy for both segments.
Overall car fuel economy rose 0.6 percent from last year to 29.1 mpg, while light-truck fuel economy increased 0.9 percent to 20.9 mpg.
In addition, van fuel economy increased 1.9 percent to a record-high 21.0 mpg mpg, although according to Wards the effect of this on the overall average was negated by a loss in market share.
Hybrid and plug-in hybrid cars also lost market share in November. Non-plug-in hybrid sales have also been stagnant for most of the year, leading some analysts to argue that they've reached a plateau.
Yet while November may have seen a downturn in new-vehicle fuel economy, overall averages are still up for 2014 as a whole so far.
Average fuel economy for the January through November 2014 was estimated at 25.1 mpg, a 1.8-percent increase over the first 11 months of 2013.