Here is my second addition to the boards: The end all be all wheel thread.
Everyone has questions on what wheels they can and cannot run. The first thing you need to think of when buying wheels is WHAT TIRE DO I WANT TO RUN? This will help you decide on what you can run for wheel specs. Most of the people like to go with a little wider tire for that performance look and a slightly lower offset wheel than stock. this is usually the best combo for fitment. Normally a little better tire than stock will be a 235 or 245 series wide since the stock tire is a 225. On most wheels you'd be able to run a 255 wide tire and still get away with it. however, the lower you go and the more aggressive of fitment you run with the wheels, the harder it is to fit everything without rubbing. The stock wheels have an offset of +52mm and you can tell they sit into the wheel well fairly far. This means we have a bit of room to move out with a lower offset wheel for that aggressive look we all want. I would normally say that the BEST offset (so long as its not too wide and not that big of wheel) is in the area of +38mm give or take a few millimeters in either direction.
The stock tire size is 225/60/16. Here are the sizes for wheels that have the same overall diameter and that are most typical for people to run:
Wheel bolt pattern
Our cars stock have a 5x115 bolt pattern. Now that's all fine and dandy, but wheels with a bolt pattern of 5x114.3 also fit. If they wobble or vibrate a lot you may need to run hubcentric rings, but that is all you would need in order to run them. Some people say that the slightly smaller bolt pattern puts stress on the studs and will snap them when driving aggressively. There are a ton of people who have been running that bolt pattern for years who would argue hardly against that point. This means that as well as some GM fitments, you can also look for mustang, eclipse, maxima, etc. wheels because they have the same bolt pattern. The thing that matters is the hub bore. So long as the hub bore is the right size then you're good.
*NOTE: THIS THREAD IS ASSUMING YOU WILL BE RUNNING A TIRE AND WHEEL SIZE THAT IS VERY CLOSE, OR WITHIN 3% OF THE STOCK TIRE SIZE (OVERALL DIAMETER). For a great website to compare tire sizes, go to 1010TIRES.COM and check out their tools.*
If you want to run a 235/245 tire:
With the tire size usually close to the stock overall diameter, you should be able to follow these offsets for easier fitment:
17" wheel, up to 8.5" wide: +32mm offset to +50mm offset
18" wheel, up to 8.5" wide: +35mm offset to +48mm offset
19" wheel, up to 8.5" wide: +38mm offset to +45mm offset
20" wheel, up to 8.5" wide: +38mm offset to +42mm offset
You want to be able to run a wider wheel or a lower offset, then continue on and learn a little more about aggressive wheel fitments.
Aggressive wheel fitments: Hellaflush and DUBs
Many people want to push the limits on the wheels they are able to put into their car. I will not go into lifting kits because I am completely against them and at no point would I ever recommend using a wheel setup that will exceed the stock overall diameter for the stock wheels (reference 1010TIRES.COM tire size calculator). There are a few ways to get wider wheel and tire setups as well as bigger wheels into the fenders without rubbing. You can either run a 'correct' offset and sized wheel/tire combo (which we all know is no fun at all), or you can get into some fender work.
With enough fender work I would venture to guess that you could run down to a +15mm offset, but this is assuming you will be rolling and pulling all the fenders on the car. Fortunately Pontiac has made this super easy for us and I can tall you right now that if you have a little patience to work on the car then you can do fender work.
Within the 'how to' section there are two write ups on rolling your fenders. I suggest you follow THIS link instead of the second link that suggests cutting the fenders (rolled_fender_2). The fenders are easily enough rolled with a heat gun and a rubber mallet. You need the heat gun because you need to keep the paint HOT to the touch, but not hot enough to bake the crap out of it. This will prevent the paint from cracking, which WILL peel. You want it to be too hot for you to hold onto, but touching it for a moment is okay. Now, temperature is relative, so don't go overboard. I have personally rolled all the fenders on the car this way and it is no harder than whacking the fenders a bunch of times. Start out hitting the fender lip SOFTLY and get harder. You have to find out how hard you have to hit the fender lip to bend it because no one can describe it exactly to you. Start in the middle and work your way out, but make sure to keep the bend even all the way across. Don't bend the middle in before the sides, go back and forth and do the WHOLE FENDER all at once. This will prevent the fender from creasing anywhere. It might take a while, but once you figure out the first fender, the other three are easy.
Rolling your fenders will give you a bunch of clearance for running a wider wheel or bigger tires in comparison to how they are stock. You gained yourself at least an inch more of room. However, if rolling your fenders does not clear the wheel and tire setup you are trying you run we have to go for more drastic measures: pulling the fenders. If you want to pull your fenders I HIGHLY suggest using a fender rolling tool (reference the 'good' link for fender rolling, its option 2). You can rent these tools online as well as buy your own if you're really looking to spend a big chunk of money on a tool youll use once. Just go to google and search 'fender rolling rool rental' and a bunch of places will pop up. Getting the tool is up to you.
What you want to do is to gradually tighten up the fender roller up and roll it back and forth on the fender. Using it to roll the fender is easy enough, but to ROLL the fenders you need to take it a step farther. Continue to tighten up the rolling tool and rolling it until you 'flare' the fender. This will give added clearance for your wheels. How much you want to pull the fenders is up to you and your needs. Once you go far enough you WILL end up creasing the fenders or making them 'wavy'. Only run as much pull as you need. Also, remember that even with the fender rolling tool you STILL NEED HEAT. REMEMBER YOUR HEAT GUN.
The biggest wheels and lowest offset wheels I have seen on a grand prix thus far are +24mm 18x9 mustang wheels. All fenders were rolled to make this setup work and the car wasn't severely lowered.
I hope this has helped everyone to help themselves with choosing a wheel and tire setup. If you stay within the normal ranges of offsets within a certain sized wheels then you will be just fine. Anything beyond these 'normal' setups and I cannot guarantee that they will not rub, nor will I guarantee they won't rub even if you are within the limits. All wheels and tires are made differently and everyone's suspension setup is also different.
Now...someone show me how hellaflush they can get!