I've ridden with 700x25 in the back (the largest my older bikes will accept) and 700x23 front. Mostly because my original rear 700x23 was cut up from debris and all I had in the closet was a 700x25. It worked fine. I haven't used that trick for awhile though. It was more comfortable on chipseal and roads with lots of loose gravel, maybe better traction on wet roads.