Originally Posted by
Juan el Boricua
Have someone measure between the fork, seat, and chainstays, substract 6-9mm from the narrowest measurement, and that should give you an idea as to how wide your new tires could be and order accordingly. And as Moe said, the inflated tire's height might cause interference with the fork crown, or brake bridge at the seatstays. Safe return trip to homebase.
Check the clearance at the brakes and at the front derailleur, which is the tight spot on two of my bikes. Check everywhere, actually.