i'll try and provide as much detail as possible
router = linksys e3000 (the dual simultaneous N one) running tomato
PC = i7 connected to router via gigabit ethernet
my mbp is the 2.4 non unibody with N. when placed in the exact same location as my MBA it gets nearly double the speed when transfering the exact same file (lets say one of my ripped mkv's at 10GB)
Are you getting faster transfer speeds on your MBA? Could it be a setting? I did not make any changes out of the box on my MBA and i get much slower transfer speds
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Also this is on my home network, not public