Originally Posted by rpenmanparker

This chart is pretty definitive and quite conservative relative to what you guys are advocating:
http://s120.photobucket.com/user/old_fool/media/bikeparts/tire-to-rim.png.html
I'm assuming a 4 mm differential between inside and outside width. That's pretty standard. Sheldon Brown admits the chart is a bit conservative, but even if you're willing to fudge one tire size larger, there is no way you should run 25 mm tires on a 25 mm outside (21 mm inside) rim. That is three sizes beyond the recommendation. Do what you want, but a little discretion might be in order.