I just put some new tires on the back of my del sol. they are: 215/40Z R17

The front ones are pretty bad. I was wondering how wide I could go on these rims? The back tires dont overlap the rim at all and look like they fit the rim good. I can get a great deal on a pair of 245/40Z R17 which on a guess is 1.1 inches wider than what I have now. Would that be overkill or just a bad idea to get them?