A car that can go 80 miles on a gallon of renewable fuel such as soy and canola would seem like an ideal solution to oil prices bumping around historic highs of $50 a barrel.

In fact, the technology already exists in the form of so-called diesel hybrid vehicles, which yoke a conventional diesel engine to an electric motor and battery to store unused energy for clean and quiet driving at lower speeds.

But automakers say such cars are unlikely to move out of the research lab any time soon, even as fuel efficiency becomes a must for more and more customers appalled by prices at the pump.

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