I own a 2017 ATS 2.0T AWD with approximately 34K miles. Last year the original Michelin Runflats were replaced because the two on the passenger side sustained sidewall damage courtesy of the Indiana Toll Road and a pothole. I went with a non runflat because of the improved quality of the ride. Recently I had to replace two of the new tires because of road damage.

I am looking into the idea of purchasing a tire with a larger sidewall, but I am not sure how large I can put on the car without encroaching into the fender area of the wheel well. Anyone with an informed suggestion or experience with this is welcome to weigh in. For reference the car now has Continental Extreme Contact DWS06 tires. The size is 225/40ZR18. Thank you!

I have a 2014 ATS 2.0T AWD running the same time in 235/40Z18 with no issues.

 

Going to a 235 won't gain you much sidewall at all - like .2". That isn't going to protect you from the pothole damage like you've already experienced. You could go to a 17" rim package and really gain sidewall, at the expense of some road feel and steering response. I run 225/50R17 winter tires on my ATS Premium RWD (FE3 suspension vs your FE2) and they fit fine. That would gain you almost an inch of sidewall. Total tire height goes up quite a bit though at .8" so your speedometer will read a touch low.

 

Only way to get a lot more sidewall and keep gearing and speedometer close to the same is to go with 17" wheels/tires. I run 18" in the Summer and 17" for my Winter wheels and tire and I bought the 17" Winter wheel/tire package from Tire Rack. Was as simple as them arriving and switching them out.

This calculator will let you see optional 18" and 17" tire and wheel packages and the sidewall height- TIRE SIZE CALCULATOR LINK