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This is pretty rudimentary and isn't always the most accurate (e.g Donnelly X'Plor MSO 650x50 measures up at a shade over 690mm dia.)
Alb-
And fat tyres are at lower pressure so sag more → more BB drop. #saggyfatties
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Different rim/tyre combos and tyres not sizing up as stated is always going to give a bit of error. My point still stands that it's nowhere near 2" difference in BB drop and that 650b shouldn't be dismissed because of it.
frankenbike
yoshy
700c = 622 mm diameter
650b = 584 mm diameter
700c x 40c = 702 mm diameter (622 + 40 + 40)
650b x 50c = 684 mm diameter (584 + 50 + 50)
Difference is 18 mm, which means your bb will be 9 mm lower. Around 1/3 of an inch.