DETROIT — Automakers are making slow but steady progress improving fuel efficiency, as the industry hit a high for the average miles per gallon of new vehicles sold in August in the United States.

A study released by the University of Michigan on Tuesday said that the average fuel economy on the window stickers of cars and trucks sold last month was 24.9 miles per gallon.

That was nearly five miles per gallon better than the 20.1 m.p.g. recorded in October 2007, when the university’s Transportation Research Institute began tracking the data.

The improvements reflect broad changes across the industry in the size, weight and engines in new models, as well as an increase in the number of hybrid and electric vehicles on the market.