OK. Let me try to answer this in parts. In some cases, I'm not sure if you are asking about the front or back so I'll give you both from what I know or measured.Originally Posted by Kermit
On my vaca, back in July, I got in the fender wells as best as I could with a tape measure and yard stick to see "what we're playin' with."
On the front, where you have the coilover struts, you'd be lucky if you could add 1/2" to the inside of the tire width before rubbing the strut assembly. It's tight. Max tire width (assuming non-lowered AND stock offset) you get in there is a 255. Anything wider in tire width or rim width, you will want to push the tire outward toward the fender - in other words - lessen the positive offset from like +48 to +40 or +38. For example, the Intense front's of 265 wide tires on 9.5" wide rims were pushed outward with a +34 offset.
Especially on a lowered car, you really don't want to Increase the overall diameter of your wheel/tire setup. This will just compound possible problems with fender lips or over bumps with weight, etc.
On the rear, you have more to play with. From the inside fender well to the inside edge (tucked) of the fender lip, you have about 11.75 of width space (298mm). The fender lip itself adds about another .875" of metal width before you then reach the exterior edge of the fender lip (320mm total). I have done a ton of research on just how wide you can go in the back and it looks like (with the right offset), you can easily get a 275 in there, 285 should be no issue too, and I personally believe I may try to put a 295 in there on mine. It's going to be tight without modifications and I'm not lowered yet. NOTE: this is not tucked! Will explain below.
There are two main and related things to watch out for in the rear. The first is that there seems to be what I'll call a "Questionable Area" of about .500-.625" on the inside of the fender well. I would avoid this area and get no closer to this area unless you want to cut and modify like the Intense car did. That area is where the fender well bump out is for the rear shock and where that brake line connector is that Intense modified. Without modifying this area, you have to choose rim widths and offsets carefully to stay away from being that close or you will hit it or rub it.
The second thing to watch out for is the lip itself. Up until recently, I thought people with wide rear tires were rolling their fender lips so they could get their tires to "tuck" into the fender, not stick under or outside of the lip, and to not rub the inside edge of the fender lip. What I found out is that while that might be true in some cases, people with wide tires are allowing the rear tire outside edge to sit under the fender lip and therefore some people are rolling so that if they are lowered or a ton of weight is placed in the car the lip won't squat down on to the tire. Note: the curved shape of the outside edge of the tire and the rolled-upwards edge of the lip helps in this regard. In other words, if corners were square and sharp, you'd have bigger problems - people would be cutting fenders.
What I figured out on mine is that I will let the wide rear tire exist under the lip (but not outside of the fender like a matchbox car) because with a stock height tire, not lowered, and the tightness of the rear suspension, that fender lip is not going to come down and hit my tire unless I was doing something drastic like flying over a speedbump with three people in the back.
Now, correct me if I'm wrong but at 605mm, isn't the stock height about 660mm (I can't remember for sure) which means that you are lowered about 2"!!! If that's the case, I think anything that resides under the fender lip will rub without rolling. So if your question is what safely can fit in the rear to clear the "questionable zone" on the inside and tuck within the fender lip even though you are lowered....would be a 275 with a tough to find +57/58 offset. I have seen a couple of 9.5" wide rims with 56 & 57 offsets but I doubt I could find them again easily. What I can't tell you because I haven't looked closely yet is this: OK, so the 275s seem to be able to tuck with the right offset...But...with you lowered, what's up inside that fender well in the top that the tire might hit when weight comes pounding down on the back end of the car? I'm not sure. Then again, your lowered suspension might also be tighter and not have much travel, which would help in this case.
Again, please triple verify all my measurements. I don't want to be responsible for you buying tires that rub.
Hope that helps.
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2009 G8 GT MGM. 
