Am I the only one on here who wants clarification to this question?
The answer really depends on which class you're running.
Absolutely, what class is very important! Here's what my research has lead to, not claiming to be definitive but just summarizing what I think I have learned so far regarding SCCA classes:
SS - Wheel sizes limited, I think mostly people here are trying to get the biggest tire they can fit on stock wheel widths. For the LSS wheel option, that is a 16x6.5et31 front for which I currently have a 205/45 up there, and for the rear a 17x7.5et38 for which I currently have a 245/40 mounted. There was some talk on page 1 of this thread about +24 offset in SS class--that is the limit of the 7mm offset change allowed for Street class in the 2015 rulebook.
SSP and SSM - Wheels unlimited, and flares allowed.
If you are not going to do bodywork it seems the popular thing here is 15x8 fronts, and I think here that +25 offset is probably a good fit along with a 225/45 tire. In the rear, 17x9 with around a +35 offset should fit a 245/40 tire.
With flares we can get more aggressive fitments. In SSM, most go for the 275mm wide or less tire bonus, wherein the SCCA permits a drop of 200 pounds from the minimum weight. With flares it seems possible to get 275 all around, I think the rears are still usually being kept at 17" wheels to keep the gearing from getting too short. Dunno what's going on with offsets here.
XP - It's getting crazy over here, Dallas is talking about 335s! I am guessing that is for his K20 swapped car that would run in XP, and not his SSM car, as I suspect running 335s in SSM would mean adding a lot of ballast to meet minimum weights! I wonder if SSP guys can go that far? It would seem like SSM with the 200# weight bonus would end up with cars that have smaller wheel/tire packages than SSP cars.