Automakers are making less progress toward federal fuel economy and emissions standards but remain in compliance, according to a report from the U.S. Environmental Protection Agency.

The new study shows average fuel economy for U.S. passenger vehicles in 2014 at 24.3 miles per gallon, which was the same as the 2013 models. Average carbon-dioxide emissions also remained unchanged at 366 grams per mile.

The EPA also named Mazda as the most fuel-efficient auto manufacturer in the U.S. in its Light Duty Fuel Economy Trends report. Mazda led the chart for the 2014 model year with an adjusted fuel economy of 29.4 miles per gallon and carbon-dioxide emissions of 328 grams per mile across its models. In contrast, FCA US recorded the lowest scores but remains in compliance.

The EPA's 2015-MY forecast shows improvement to 24.7 mpg and a decline in carbon dioxide emissions to 360 grams per mile.

Mazda achieved the highest fuel efficiency for the third consecutive year without offering a hybrid or electric vehicle. Mazda credits its SkyActiv technology, a suite of component and engineering technologies that cut vehicle weight and improve engine efficiency.

Read the full report here.