There are many threads asking for the widest wheels/tires in this section. There are many variables to the answer. Also your question, widest tire on the stock offset, does not make much sense. Offset is the measurement of where the wheel is bolted to the hub relative to the centerline of the wheel. 0mm offset means the wheel is bolted right in the centerline of the wheel. 40mm is common for Mustang wheels, and it means the hub bolts to the wheel 40mm closer to the face of the wheel from centerline.
Sorry if the way I asked the question was not clear. I fully understand the concept of offset. That's is why I specifically asked widest tire keeping the stock offset. I did not find this using the search. I do not want to run a wheel that will require a spacer or longer wheel studs. So if the can maintain that +40 offset and run a 295 or 305 up front that would be great if not I need to reconsider my plans. Either narrower tire or a change in offset and get wheel studs or spacers if needed and whatever else. Hope this clears up the questioin and makes sense now. Thanks.
Offset relies on the width of the wheel. Stock offset means next to nothing by itself. You need to maintain clearance from the inner fender and strut plus the outer fender and when changing the width of the wheel(to fit 295/305) you will need custom offsets.
There is a member/vendor here(Sock, from Dusold Designs) running 18x11 wheels with 315/30/18 tires at all 4 corners. Custom offsets are required. Wheel spacers and longer studs are dependent on the design of the wheel itself(spoke design, caliper clearance). Plus, to run 315's up front he has very short tires(<26") and also has coilovers(only recently) which increase inboard wheel clearance.