Carlise (Carlise wheel that is...) who I buy alot of steel outers from make a few different 17" steel outers. Thier customer service is terrible is the only problem (I cringe every time I have a big order to place, but they have a captive market), but I do have thier sizes handy.. they do a couple different ones.
A 17" standard is 15.470" ID
A 17" custom is 14.228" ID

So obviously the standard are going to use a larger center than a normal 15" wheel so you would get more brake clearance that way.


The 20" steel outers are done by another company I deal with...I think they might do 17's and perhaps 18's as well. Don't know thier center ID sizes though off hand on the 20" ones at least.