If you go back a week or two on the message board you will find a thread that talks about this exact issue.
Short summary:
anything between 38 and 42 (considering you don't have a extreme drop) would work without any problem if you go lower, then the wheel might rub on the exterior side of the wheel compartment , but if you go higher the wheel might rub on the interior of the wheel compartment.

click here for the thread