I'm planning to buy new rims for my Sunfire as the current ones are heavily worn and their restoration costs more than new ones.

What is the maximum rim size for this car? The current is 15x7 but I want to switch to 17x7 or even 18 if it'* possible.

By my info, you have: 195/65/R15

Optional from the manufacturer was: 205/55R16

You can go bigger, but the ride will get rough.

Yeah, that is correct.

If I go for a 18x8 rim I will prolly have 215/40R18 tires.
I am concerned mainly about the tire width that it will be too wide and will just splash dirt all over the car.

Can you give some tips on how to measure the maximum width? (I'm new to wheels, never bought any)

I think if you go to 215s you probably won't have any issue with rubbing etc. if you keep the tread diameter the same.

Your 215/40R18 size should be fine. The diameter is 0.8% smaller than your current tires. If there is any tendency for a clearance issue with the wide tires, the slightly-smaller tire diameter will slightly help offset that.

The other thing you need to be concerned with is the wheel offset. You need the same wheel offset as you currently have.

. . . and then the bolt pattern has to be the same as well.

Thanks.
Yes, the bolt pattern is correct (5x100). I also read that is okay for the center bore in a wheel to be bigger than the center hub in the car, is that right? The center hub in this car is 57,1mm which makes it really hard to find any nice wheels that would have the exact same center bore.

Also, what do you mean by tread diameter? Do you mean the whole wheel diameter?
What about 225/40/R18? Isn't it reccomended for a 18x8 rim?

As for the offset, the current one is ET40, would it be a problem to operate in a range ET48-ET30? Many of the wheels I am considering have the ET at 45 and some at 35.

I would not rely on a wheel that has too big of a center bore on most vehicles built in the last few decades. Lug nuts and studs are designed to make the wheel into part of the hub, not to support the weight of the vehicle on the wheel. They make adapters to adapt a smaller hub bore to a larger wheel bore.

Correct.

Many tire sizes can appropriately be installed on a 18x8 rim.

It started at 195mm
Factory could have done 205mm
You were looking for 215mm
Now you're talking 225mm

By Post #10 you might end up here:


LOL

I wouldn't keep increasing section width unless you are positive, through measurements, that you can fit that size of tire without rubbing etc. Also, I'm not sure how good the roads are in your area, but the wider you go, the more you can feel. In general. Also the shorter the sidewall, the more you can feel. One benefit of wider tires is they don't fall into potholes as easily.

195/65R15 is 638mm diameter
215/40R18 is 630mm diameter, so 0.8% smaller and 2cm wider than 195/65R15
225/40R18 is 638mm diameter, so 0.4% taller and 3cm wider than 195/65R15

Also, keep in mind that the more rotational weight that you put out there, the more your performance will be impacted. It is possible that if you currently have steel rims, you could go to aluminum taller rims and come out lighter. Depends on what you choose. Maybe you already have aluminum rims, so you'd have to go to magnesium or carbon fiber to go lighter. Same goes for tires.

Another thing to keep in mind: Look for what size tires are available in your region. Maybe good ol' CathedralCub convinced you to go with 215/40R18, but they are extremely difficult and/or expensive to get a hold of. Then you'd want to go to a different size.
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I would recommend keeping the same exact backspacing as factory, and I would double-triple recommend keeping the same exact backspacing as factory as you go to wider section widths.

While we're here: You could go easily measure for tread versus fender interference as it relates to suspension travel. Figure out how much tread is missing, use something to simulate that difference, then measure half of the larger section widths and see if they get close to the inner edge of the fender. Same goes for steering. Park the car such that you can get under it. Steer all the way right and measure clearance between the tire and frame, body, strut, etc. Then steer all the way left and do the same. If there is at least 1cm of clearance, you might be okay and/or might have some rubbing in extreme circumstances.