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It can be bad for the converter if one runs too low on fuel and the engine misfires while gasping for the last amount of gas. Another reason for not running low during the summer is that the gas gets warmer returning from the engine(fuel pump supplies gas at 100% capacity and unused portion returns to tank) and having more gas allows for less chance of vapor lock(although less likely to happen with today's fuel injected cars). During winter, running with more gas lessens condensation in tank.