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Optimal tire pressure is the key here, I think. A narrow tire requires a higher pressure. Pretty all eco-cars are delivered with narrow rock-hard tires.
But is the label as reliable as it comes to fuel savings. RRC is only one part of the story.
Again, in my case, the 205/45 gets a better score than the 185/60. But I can imagine that a narrower tread weighs less, and also causes less air resistance.
A narrow tire at maximum pressure (= often higher than the wide one) should be more fuel efficient. Despite a higher -theoretical- rolling resistance
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