The overall diameter of a 170/60 is a 2% larger than stock, while a 170/55 is about 1% smaller. No big deal either way.From what I was told the other day, if you go one step wider, you go one smaller on the wall. So stock we're 160/60. If you go wider, you should go 170/55. That will keep the geometry the same and keep the rear from raising.