Hello Bigmike
I did a lot of research before I went with 255/85/16's, so hopefully some of this information will help you out.
255/85/16's have a diameter of 33.02, and a cross section of 9.87
285/75/16's have a diameter of 32.08, and a cross section of 11.30
I went with a 1.25 inch spacer in order to maintain the stock distance between the tires. For the 285's you would need a 2.00 inch spacer to do the same.
Hope that helps.