The strong sales of the Ram 1500 Diesel has contributed to the national improvement in fuel economy for new vehicles.

With gasoline prices inching upward this summer, the fuel economy of the new vehicles sold in the U.S. is continuing to improve compared with historic figures, the University of Michigan reports.

Despite a slight dip in June, fuel economy of new vehicles sold in the U.S. has exceeded 25 miles per gallon for the fifth straight month, according to researchers at the University of Michigan Transportation Research Institute (UMTRI) responsible for the monthly survey of fuel economy trends.

The average fuel economy – window-sticker values – of cars, light trucks, vans and SUVs purchased last month was 25.5 mpg, down from a record-high 25.6 mpg in May. Vehicle fuel economy is now up 5.4 mpg from October 2007, the first full month of monitoring, noted UMTRI researchers Michael Sivak and Brandon Schoettle.

In addition, UMTRI’s national Eco-Driving Index (EDI), which estimates the average monthly emissions generated by an individual U.S. driver, rose slightly. The EDI takes into account both the fuel used per distance driven and the amount of driving – the latter relying on data that are published with a two-month lag.

The EDI stood at 0.80 during April, up from 0.78 in both February and March. The index currently shows emissions of greenhouse gases per driver of newly purchased vehicles are now down 20%, overall, since October 2007.

The federal Energy Information Administration, which in recent weeks reported gasoline prices had increased in the wake of turmoil in Iraq, said the prices gasoline and diesel fuel have leveled off over the Fourth of July holiday.

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During the weekend, consumers paid the highest gas prices in six years for the Fourth of July, with most drivers paying 15-20 cents more per gallon than expected and an increase of 1.9% over last year, according to AAA.

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However, AAA also said it appears that prices at the pump won’t be as much of a deterrent to Americans this summer as previously predicted.

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Gasoline prices in the U.S. have been rising despite a decline in crude oil prices, which hit a three-week low as tensions in Iraq eased and supplies of crude around the world improved.

Analyst are suggesting that gasoline prices are unlikely to retreat much from their current level of around $3.60 per gallon, on average, for the foreseeable future.