2nding UA's point, different brands/models of tires seem to have slightly different diameters and will be tight or loose fits. I once got a Continental Traffic tire mounted on a Mavic Crossland UST wheel and it was such a tight fit that neither I nor the owner (not a mechanic) of a local bike shop couldn't get the tire off the rim. I felt much less sheepish after the mechanic at least had to use tools to get it off.