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The white bars show the efficiency of the vehicle; the black bars show overall efficiency of the fuel and its vehicle for various fuel/vehicle types. The overall efficiency of the diesel / hybrid system (third black bar from top) is greater than today's hydrogen fuel cell systems because it is far more efficient to produce diesel fuel from oil than it is to produce hydrogen fuel from natural gas, the most likely source. Even the highly efficient fuel cell vehicle proposed by the DOE for 2010 (white bar at bottom) ends up being just 18 percent more efficient (black bar at bottom) overall than current diesel / hybrid prototype.

Sources: Industry, academic and government studies.