Trucks are hot sellers these days, in part because they are getting better gas mileage than ever.

Using lighter materials, turbochargers and other methods, automakers are taking to make new cars and trucks more fuel efficient continue to pay off as the gas mileage average in the U.S. hit a record 24.1 miles per gallon last year.

Tempering that good news is the fact that the year-over-year gains are slowing, according to the Environmental Protection Agency (EPA). The 2013 result was 0.5 miles per gallon better than 2012, but was significantly less than the 1.2-mpg improvement from 2011 to 2012.

However, due to a sales increase in full-size pickups and SUVs this year, the agency is predicting a much smaller improvement from 2013 to 2014: 0.1 mpg. Sales of light trucks are up 16.5% this year and account for 51.3% of overall sales through September. Last year, they were 49.3% of overall sales. Conversely, car sales have risen just 2.3%, according to Autodata.

The sales jump is due in part lower overall gas prices and improvements in the fuel efficiency of large vehicles. The government predicts the average price for gas will be its lowest since 2011 at $3.45 a gallon.

Mazda's 2014 Mazda6 sedan already meets 2019 fuel economy standards. The Japanese maker boasts the highest average fuel economy for its line-up in the U.S.

However, according to researchers at the University of Michigan’s Transportation Research Institute, the average fuel economy based on the mandatory window-sticker mileage figures of new vehicles sold in the U.S. in September stood at 25.3 mpg. While down 0.5 miles per gallon from the value in August, it’s higher than what the EPA is predicting for all of 2014. However, they note that the rate of mileage improvement is dropping in their research as well.

Another factor in the expected drop, according to Chris Grundler, head of the EPA’s office of transportation and air quality, is the fact that Hyundai and Kia delayed the introduction and sale of higher-mileage new models this year. Taking Hyundai and Kia out of the equation, the improvement rises to 0.4 mpg, still short of the 2013 gain, according to the report.

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Since the EPA began tracking the changes in 2004, new car and truck mileage ratings have improved 5 mpg. The EPA noted the mileage of truck-based SUVs rose 27% from 2004 through last year, which was the biggest increase of any segment. Cars and car-based SUVs each saw a 23% improvement.

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Agency officials said they still expect the industry average to meet the 54.5-mpg mandate for corporate average fuel economy for 2025.

In fact, Grundler said the auto industry is ahead of what the EPA expected at this point, and he expects improvements to vary from year to year depending on the new models that are introduced.

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He noted that the aluminum-body Ford F-150 pickup, due out later this year, could have a significant impact on the average because it’s the country’s top-selling vehicle.

Mazda is tops among automakers with an average of 28.1 mpg. Honda was second at 27.4 mpg. Due to the fact they sell more trucks and SUVs than other automakers, Chrysler, General Motors and Ford found themselves at the bottom of the rankings.