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FWIW, I recently switched out the heavy RFTs on my e91 328 for very light Conti DWs. I think the weight were around 27F, 29R for the RFTs and around 21F, 24R--getting close to 25 pounds of rotating weight as far from the hub as possible. My mpg went up by anywhere from 0.5mpg to 1.2mpg per tank. Nothing like what you are seeing. Of course mine were stock sport pack sizes...
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