OK Dougs, now I understanding what you're saying about summer vs all-season tires, but even though they are listed as the same size, in reality they really aren't.
That's because you're talking about Michelins vs Continentials, and there will always be a different hight, or width to contend with simply because there is no "Industry standards" that dictate that tires have to meet any exact specifications, for comparable sizes.
Bladecutter, you're absolutely correct about the tires weight effecting the gas mileage. But I would think that would be more noticable in stop and go traffic instead of on the highway. A heavier tire would cause the engine to use more gasoline just to get it moving than a lighter tire would. But on the highway at a constant speed, the heavier tire may actually improve the MPG thanks to a "flywheel" effect, creating more inertia.