The national average fuel economy for new vehicles sold in May increased to 25.6 mpg according to the University of Michigan Transportation Research Institute, while AAA says that gas prices are essentially holding steady.

The most recent data from the Institute's Eco Driving Index shows that the May average sales-weighted fuel economy for light vehicles is increasing 0.4 mpg from April, and 5.5 mpg from October 2007, when researchers started collecting data.

The institute calculates averages the combined city/highway EPA fuel economy ratings displayed on window stickers and sales-weighted fuel economy from monthly sales of light-duty vehicles.

Researchers Michael Sivak and Brandon Schoettle have said that the fuel economy data was available for 99.8 percent of all vehicles purchased in May.

AAA Daily Fuel Gauge Report says the nationwide average price for a gallon of regular gas is at $3.66, compared to $3.67 in May.

"While it is impossible to predict the exact price of gasoline, we can guarantee that millions of Americans will pay high prices as they hit the roads this summer," said AAA spokesman Avery Ash in a statement, according to the release. "Expect a feeling of déjà vu with gasoline costing about the same as last summer."

AAA expects fuel prices to remain in the $3.55-$3.70 range during the summer but said that damaging hurricane season, major refinery disruptions, and geopolitical concerns could raise prices higher than expected.

Average national fuel prices last summer ranged from $3.47 on July 6 to a high of $3.67 on July 22, according to AAA.

Gas prices ranged from $3.58 per gallon from Memorial Day through Labor Day in 2013.

The most expensive year for driving in the summer was 2008. Prices averaged $3.95 per gallon that year.

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