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305 will fit on a 10inch rim, but it won't have the sidewall support of a 275 or 285 and, thus, will feel mushy in the corners. They aren't light, there was a thread a while back about them, I remember being very unimpressed by the weight... I wanna say they were around 24lbs a piece.
Those frickin rims are some bucks and for that much they should be super light. Paying for the name but no different than buying other rims like Foose etc.