Impact on MPG
Assume that the width of the tire on the 17" is the same and the overall diameter of the tire is also the same, the impact on MPG would be very minimal. Some pros and cons:
17" over 15"
- shorter sidewall: hasher ride, better control in aggressive turn (in a Prius?)
- slightly more weight (assuming same alloy/rubber material): minor impact on MPG (<1%), longer stopped distance (sligtly), slower 0-60mph. All those were reported by an auto mag about a year ago under a strictly controlled comparison. The differences are all very small, if any.
Normally, when people move up to 17" from 15", they also increase tire width. That is where you would get a major hit on MPG due to more aerodynamic dragging and friction. When I up-sized my BMW540iA's tires from 225x55R16 to 235x45R17(F)255x40R17(R) (almost same diameter), I have been seeing about 5% hit in MPG. However, the handling is much improved at turns. In my Prius, I don't see the points.