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I don't think it makes any difference at any speed unless the driver can't account for the increase in KE via having larger coasting distances. I mean, less mass is good for normal city traffic in that marginally less energy is lost when everyone gets on the brakes from some speed. But if we stay off the brakes, having more KE at some given speed isn't an issue at all since it'll just take us slightly farther as we coast down. IMO, lighter wheels just aren't worth the cash to those looking to get better mileage compared to just about anything else.
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