If you're going to go with an 18'' wheel then you should definately go with the 215-35-18 size tire. If you ask the shop that you're getting your wheels from to do a tire size calculation, you'll see the a 215-35-18 is actually smaller than the stock 195-60-15. It's only about .3 of an inch so its not really that big a deal. But you should not lose any power, and depending on what type of rim you choose, it might ever be lighter than the stock rim.
Hope this helps.
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