For automotive applications, it is necessary to maximize the fuel conversion efficiency of a PEM direct-methanol fuel cell (DMFC) over the broadest possible dynamic range of power. The research reported here critically examines the efficiency of the DMFC stack when operated over a broad power range. This research establishes a basis for a control strategy that simultaneously: optimizes DMFC fuel conversion efficiency versus power level, leads into a system level optimization of efficiency vs. power, and provides an operational strategy for controlling a direct-methanol fuel cell for maximum fuel efficiency from minimum to maximum power demand.First, there is an explanation of the experimental conditions used to obtain the DMFC experimental data that is reported and analyzed. Next the DMFC methanol crossover phenomenon is discussed and characterized. Then the conceptual framework for the optimization of fuel conversion efficiency is presented. Finally, the optimized fuel conversion efficiency is viewed in terms of the conventional voltage efficiency and fuel utilization parameters traditionally used for direct-hydrogen and reformate fuel cells.The primary conclusion of the research is that, at a given DMFC fuel consumption rate, the DMFC power density and fuel conversion efficiency is maximized by simultaneously controlling both the concentration and flow rate of the methanol fuel. This yields an optimized efficiency curve (vs. power level of DMFC operation). An additional optimization of the air flow and pressure conditions is clearly also possible, but is not explicitly developed as part of the research reported in this paper.A key feature of the optimized fuel efficiency curve is its relative flatness versus power density (e.g., greater than 30% efficiency over a range from about 70 to 230 mW cm-2). The major operational result is that it is conceptually possible to optimize the conversion efficiency of a DMFC power system by manipulating the methanol fuel feed stream as a function of the system power demand. Practical application of such a strategy, of course requires a variable concentration, variable flow methanol fuel control technology.