You'd be fine just keeping what you have on there, the 215 is just a slightly wider tire.. However, because the aspect ratio is the same, the sidewall would be slightly bigger on the 215 tire. But, if you're not experiencing any issues such as unwarranted abs operation or lights then you're fine just keeping what you have.

When ever you replace the tires on an axle, just put the 215s on it.

Yes, ideally you would have 215/70-15's on all four corners of your Buick. The 205 width tire has a lesser contact patch and a smaller load carrying capability. It is entirely fine to buy tires in pairs. Go get a quality pair of 215/70-15 sizes tires but have them mounted on the rear axle. Keep the two 205 width tire that have the most remaining tread and place them on the front axle until they can be replaced at a later date.

205 will give you worse economy and false speedometer readings, you should put them on the front if it's front wheel drive

I bought the car used with 205/70/15 on them......buick recommends 215 70 15

so the only difference is 205vs 215

id like to put the 215 on so it has whats suppose to be on them. the back tires are fine. I just wanted to replace the front tires. but it would have to be with a 205 because of the back. what bothers me is 215's were suppose to be on them.

will I noticed any difference ? between the 2? any advantages over on or the other? thanks