Joined: Jan 2010
Posts: 9,797
Likes: 1,763
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
27.5 refers to a mountain bike-width tire mounted on a 650b rim.
With the wider tire, the outer diameter climbs from "26 inch" to "27.5 inch", for tires listed as 1.5" and 2.0" wide, respectively.
Not sure how accurate or universally consistent those quoted dimensions really are. The bigger tire also has tall knobs which add to it's height.