The University of Michigan study indicated the gains made in reducing greenhouse gas emissions since monitoring of the cars' performance began at the university's Transportation Research Institute.
Years of research into the cars' success with cutting back emissions have provided researchers with new tools, the Eco-Driving Index being a good pointer.
The EDI estimates the average monthly greenhouse gas emissions generated by an individual U.S. driver. That figure stood at 0.87 in December 2011 and indicated a 13 percent improvement from October 2007.
The EDI takes into account both vehicle fuel economy and distance driven, the latter relying on data that are published with a 2-month lag.
The fuel efficiency of all the cars sold in the United States is rising, the institute said.
Vehicles purchased by consumers in February averaged 23.7 miles per gallon, up from 22.7 mpg in February 2011 and from 20.4 mpg in February 2008, the institute's findings showed.
The Los Angeles Times reported this week 24 percent of the vehicles sold by Ford Motor Co. in February were small cars, Ford analyst Erich Merkle said. That is a quick jump from December, where less than 20 percent of the vehicles sold by Ford were in the small car segment.
With gasoline prices steadily on the rise, 70 lawmakers sent a letter to the Commodity Futures Trading Commission demanding a tougher stance on oil speculators.
The national average price of gasoline in the United States jumped nearly 30 cents in February, climbing from $3.475 per gallon to $3.767 per gallon, AAA said.
Earlier findings by the institute indicated that light-duty automobiles -- mostly cars -- give motorists up to 1.7 miles per gallon more on the road than vehicles bought before 2008.
"Recent Fuel Economy Trends for New Vehicles in the U.S." reported on gains made as a result of automobile design and manufacture changes made in recent years.
The report by Brandon Schoettle and Michael Sivak indicated fuel savings in most types of vehicles bought during the 2008-12 period.
Vehicles using diesel and gasoline, the two dominant fuels, showed marked improvements. While gasoline engines showed the smaller increase, gaining 2.3 miles per gallon, diesel engines showed a much larger improvement, with gains averaging 9.8 miles per gallon.
The less prevalent vehicles using battery-electric and hydrogen fuel-cell technologies are rated about 2-to-4 times as efficient as gasoline and diesel.
The researchers looked at improvements in fuel economy achieved since 2008 in a range of vehicles, comparing savings in cars and light trucks.
They examined fuel efficiency by vehicle size class, transmission type, number of engine cylinders, drive type, fuel type and hybrid versus conventional vehicles.
The research on fuel savings comes amid continuing concerns on both energy dependency and greenhouse gas emissions.
Last year the U.S. Environmental Protection Agency and the U.S. Department of Transportation's National Highway Traffic Safety Administration issued a proposal extending a national program to further reduce greenhouse gas emissions and improve fuel economy for model year 2017 through 2025 light-duty vehicles.
The EPA proposed national greenhouse gas emissions standards under the Clean Air Act, and NHTSA proposed Corporate Average Fuel Economy standards under the Energy Policy and Conservation Act as amended by the Energy Independence and Security Act.
The standards proposed would apply to passenger cars, light-duty trucks and medium-duty passenger vehicles, covering model years 2017 through 2025.
The proposed standards are projected to require limits of 163 grams per mile of carbon dioxide in model year 2025 -- about 54.5 miles per gallon if the vehicles were to meet this carbon dioxide level all through fuel economy improvements.
The standards are meant to be applied across average industry fleet and include all passenger cars, light-duty trucks and medium-duty passenger vehicles.
New national rules being developed by the Obama administration will result in new cars averaging 54.5 miles per gallon by 2025, double current fleet average for new cars.
Last year the administration also unveiled fuel-efficiency standards for heavy-duty trucks.
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