[Overdrive]

It may be a model year specific thing, then. I can't remember what year yours is, but my 97 has 7in fronts and 8.5 in rears (made a typo earlier and put the .5 in the wrong spot, my bad). I've never known the 986 to have anything different for 17s. I would also say don't judge the wheel width by the tire width, since (to me, at least), the rim sits a bit deeper inside than the outermost part of the wheel, so you may very well have a half inch on either end that's just rubber.

Since the 996 had the same wheels at that same time, perhaps just with more width (just speculation on my part, not 100% sure), it could be that you have the 7 or 8.5 in front and the 9inch rear off of a 996. The style's exactly the same, it's just a deeper wheel.