Will American auto manufacturers ever be able to engineer the world's most efficient vehicle?

According to Motor Trend's latest list of the 15 most fuel-friendly vehicles, many of the dominant manufacturers continue to hold onto their top spots in the fuel efficiency hierarchy. Once again, vehicles from Toyota, Honda and Nissan will provide drivers with the greatest relief from high gas prices.

American vehicles did appear in the top 10, however. The Mercury Mariner and the Ford Escape tied for sixth place (alongside the Mazda Tribute). All of these vehicles are hybrids but, ironically enough, they are all SUVs as well. Americans are left wondering when Detroit's "Big Three" will deliver an ideal sedan.

In January 2000, General Motors Corp. unveiled the Precept -- a vehicle that, according to the company, was capable of 80 miles to the gallon with a 500-mile range. The New York Times referred to the diesel-electric hybrid as "probably the most expensive single car ever built." The Precept has yet to go into wide-scale production.

This year, GM did offer two efficient sedans: the Saturn AURA and a hybrid version of the Malibu. According to the GM Web site, both vehicles are capable of a combined 28 miles per gallon. Still, they fail to appear on the Motor Trend list. According to Motor Trend, "All vehicles on the list had to be widely available now, so it is comprised of 2008 models only, and the cars that require pricier premium gas (possibly eliminating their fuel-economy advantage) have been noted."

Regardless of these factors, American car shoppers will still emphasize a vehicle's payback period, or the time necessary for an investment to pay for itself. The costliness of new automotive technologies paired with their high demand in the marketplace means "going green" is going to cost a lot of green too -- a notion that will forever undermine the good intentions of good gas mileage.