New light-duty automobiles entering the U.S. market are more fuel-efficient and give motorists up to 1.7 miles per gallon more on the road than vehicles bought before 2008, a University of Michigan study indicated.

"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 the past five years.

The report by Brandon Schoettle and Michael Sivak from the university's Transportation Research Institute 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 two to four 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.

Last week California's Air Resources Board approved historic rules that require 15 percent of new cars sold in California by 2025 run on electricity, hydrogen or other systems producing little or no smog.

The "advanced clean car rules" also require automakers to cut greenhouse gas emissions by 50 percent on all new vehicles by 2025 and tailpipe emissions of soot and smog by roughly 75 percent over the same time period.

The greenhouse rules are similar to new national rules being developed by the Obama administration and 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.