If I get wider 19's for the back do I need to get new tires? or will the stock ones fit

If I get wider 19's for the back do I need to get new tires? or will the stock ones fit

Most MFG have recommended rim size for each tire size. If your tire size is 245/40/19, rule of thumb would support any rim between 8 and 9.5 inches. If you are buying 10's, you want to ditch the 245's anyway. Here is a handy calc for getting diameters, etc.... FYI.

http://www.1010tires.com/tiresizecalculator.asp