I had bought 265/65/17 tires with 8/32nd threads left where the price too good to pass up. I put it on the front and it was way too close to the upper ball joint/ control arm area. I then put them in the back and all is well.
I've been driving with two different tire sizes for about 20K miles with no problems but my speedo is 02/mph off though.
You could tell that the tires were different sizes when the larger was in the front, but now they look much better in the back. It fills up some of the rear fender space.
I cant tell if my mpgs were effected, but if so, it would be ever so slightly.