Strange, thought 650b was 27.5" But I have found it very difficult to find more road going type of tires for this size. My 11 year old son has this size on his GT Backwoods. Wore the offroad tires it had completely out since he rides on the road so often.

But did find Kenda tires at Dicks Sporting Goods in this size and are much more road worthy. Kenda K841A 27.5" Bike Tire | DICK'S Sporting Goods

These tires actually look better on the bike than the picture.