Get a $95 Speedo Healer, and you will have an accurate speedometer with any sprocket combo you can cook up.
I ran a 17 for a while when I was working out of different centers. I'd commute an hour one way. Long distances like that are all the 17 is good for. Fuel mpg's did not improve.
Stock gearing is nearly perfect. But I still like to play with the 15 once in a while.