As I said above the problem is the seat stay bridge clearance. It's pretty tight there but I haven't tried 47mm. I did look into getting 650b wheels and contacted Specialized UK they replied that it wasn't advised as it would lower the BB. But I think the larger (47mm) tyres I was planing to fit would compensate for the smaller rim size and so should be similiar overall dimension to 700c with 38/40mm tyre. It seemed a good solution to fitting wider tyres. Anyway I found the 27.5/650b wheels available at the time to be expensive so I didn't take the risk & fitted the WTB Nano 40mm/700c tyres instead.