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There was a discussion about this a while back. Here's the deal: the downside of taller is that ride height is [very slightly] higher, and you have a bit more frontal area. On the other hand, if you do a lot of highway driving, a taller tire will simulate taller gearing, lower cruising rpm. The lower rpm could save fuel. And now the kicker: if you do a lot of in-town driving or have an underpowered engine, the simulated higher gearing may cause more downshifting to be necessary, resulting in lower FE.
I think some hard data is needed to display which of the above factors carries more weight (which reminds me that taller may mean heavier ie. more rotating mass). I don't think there appears to be an across-the-board winner. Rather, there are numerous factors to be considered, some of which are admittedly splitting hairs, but they are worth mentioning nonetheless.
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