650b x2.1 is fun, a lot of fun….on single track and muddy gravel in February. It is a hell of a lot of work, and heavy, and slow if you’re course doesn't have much of those things in it, however. 700x40 is a a nice split between x38 and x42…if you’re going to for time, ride the 700s. IMO, the only way 700x40 is underbiked is if the route is laden with big chunky rocks (that wish they were boulders) and rooty single track. Enjoy!