Youd be better off checking impala forums because a lot goes into play with rims and tires fitting on a car. Different treads, offset, tire/rim width and more all affect clearance.

Your tire / wheel combo is 2 circles. It don't matter how big one circle is as long as the other circle fits inside of it.

You say 225/17 and 225/20, you're missing a number in the middle, what is that number? I'm assuming 60.

A 225/60/17 is a 17.6X8.86R17 in standard measurements. A 225/60R20 is a 30.6X8.86R20 in standard measurements. You're almost doubling the tire size there, so that will not work. The tires you'd be looking for are 275/35R20. That is the exact same width and height as your current tires so those with 20" wheels will fit. Now where they may not fit is dependent upon how much offset and backspacing you have. The lower the offset, the more the tire will stick out and rub on things like your fender and bumper. The higher the offset, the more it will sit under the car.

If you get wheels that have the same offset and backspacing as your wheels right now, you'll have no problem.

EDIT:
It will also depends on how wide your wheels are now vs how wide you want them to be.

Here is a couple pictures explaining offset:


Extreme negative offset:


Positive offset:

002 wrote Your tire / wheel combo is 2 circles. It don't matter how big one circle is as long as the other circle fits inside of it.

You say 225/17 and 225/20, you're missing a number in the middle, what is that number? I'm assuming 60.

A 225/60/17 is a 17.6X8.86R17 in standard measurements. A 225/60R20 is a 30.6X8.86R20 in standard measurements. You're almost doubling the tire size there, so that will not work. The tires you'd be looking for are 275/35R20. That is the exact same width and height as your current tires so those with 20" wheels will fit. Now where they may not fit is dependent upon how much offset and backspacing you have. The lower the offset, the more the tire will stick out and rub on things like your fender and bumper. The higher the offset, the more it will sit under the car.

If you get wheels that have the same offset and backspacing as your wheels right now, you'll have no problem.

EDIT:
It will also depends on how wide your wheels are now vs how wide you want them to be.

Here is a couple pictures explaining offset:

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Extreme negative offset:

Click to View Content

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Positive offset:

Click to View Content

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Well said for sure! For starters i would go with a real low profile tire, you can probably make it work but your gonna need to roll the shit out of those fenders and then get a 30 profile tire so nothing scrapes