You can use a 225-50R15, it will fit on a 6.5" wide rim. You aren't asking too much of the tire to do that, recomended rim width for that size tire is a 6-8" wide rim. You might be able to get away with a wider tire for that rim size it will just come down to clearence and availability. A 225-50R15 has some good choices for AutoX tires.
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