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when I have done them on my Weber I went 3 hrs for about a 8lbs, then when I sliced I throwed the slices on the gas grill for those that didn't like med rare, best I remember I ran 275-300 with charcoal and hickory .
You should ask @Bearcarver (resident expert at rib roasts) about this to make sure but I asked the same question b/c I had a 5 bone roast and asked how much longer does he think it would take and surprisingly, it doesn't take that much longer than the 5 and 6 lb roasts he does. My 11 pounder took about 5.5 -6 hrs which I believe was the same time it took him to do half the size. It has to do with the roasts not being wider, just longer (as in amount of bones)