You might be able to get away with putting that size of tire on a 15x8 but more than likly it is going to crown. Mount one tire and see but if it where me I would look for a set of 15x10 like JoshH said. You can usually get away with a 4-5" difference between tire and rim size and be safe anything bigger and the tire tends to crown. Hope this helps.
Like the guys said above that is probably too wide. I've always been told 4" difference between the wheel width and tire is the max. But if you go with really tall skinny tires you can get away with fudging on that rule a bit.
like most people say. thats too wide for proper fitting I would'nt go over 12.50 wide the tire will crown wear bad and ride horrible on the road.
Every now and again I work on tires and see this problem. good luck with that wide one!!!