I was really curious about Jan Heine's findings on wheel size/tire size combinations so I ordered the relevant issue of Bicycle Quarterly. I don't remember the specific numbers but he and his fellow testers found that skinny 650B tires just didn't handle very well. Roughly, 700c = skinny tires, 650B = medium tires and 26" = wide tires for what he terms "optimum handling". I know that low 30's is the widest he recommends for 700c because that's what I ride. 650B might have been upper 30's to lower 40's but not sure.