The most fuel efficient 2008-model cars available to consumers are hybrids, according to new fuel economy numbers issued by the EPA. The Toyota Prius, with a combined highway/city mileage of 46, and the Civic Hybrid with combined mpg of 42, are the only vehicles to break the 40-mpg mark. The rest of the top five most efficient vehicles are also hybrids: the Nissan Altima Hybrid, Toyota Camry Hybrid, and two-wheel-drive Ford Escape Hybrid, the only SUV at the top of the list.

The 2008 fuel economy figures for hybrids are about 20 percent lower than last year’s numbers, based on new testing procedures designed to better reflect real-world driving conditions, such as high-speed driving, use of air conditioning, and cold weather. Industry observers speculated that hybrids—commonly criticized for having exaggerated official mpg numbers used on window labels—would tumble when the new testing system took effect. Despite the adjustments, hybrid gas-electric vehicles remain as leaders in fuel efficiency.

The top ten highest ranked vehicles for mpg include three non-hybrids: the manual and automatic versions of the Toyota Yaris, a subcompact, and the manual Toyota Corolla.

According to the EPA, diesel-powered vehicles typically get 30–35% more miles per gallon than comparable vehicles powered by gasoline. Yet, the most efficient vehicle in the EPA ratings, the Mercedes E320 Bluetec, was rated at 23 in the city and 32 on the highway—well below the most efficient hybrids.

Automakers are working on number of advanced technologies—such as plug-in hybrids, battery electric vehicles, and more efficient gas-engine systems—to produce major boosts in fuel economy. But the timing is uncertain for the application of those technologies in production vehicles available to consumers. In the meantime, it’s all about hybrids for car shoppers searching among 2008 models for maximum mpg.