Yeah, that sounds about right. The wheels I have are 22" w/out the tires, and the bead seat is about 1/4" less on each side, so that'd make it 21.5", which is 546mm. Hmm. Now, to find tires that are going to fit. Hard to tell with some of these, as they don't always list the bead diameter.

The 24 x 1-3/4 I have are literally a couple inches too small, bead diameter-wise, when you're trying to put them on the rim. If I lay the tire down on the ground, and set the rim on top of it, however, it does look pretty close. There's just no way I can get them on there, so I believe you're right - I just don't want to order yet another set of tires, and have them not fit. Heh.

Thanks for your help, and please - throw any more ideas you may have my way!