I have my doubts that this will work / work well on a Prius. It has a CVT and as soon as you're off idle your engine is already at optimal load. Going to a larger tire would only help during times where part throttle is needed which really isn't that often in the Prius. I think the added weight would counteract the benefit as well as has been seen in the Prius with 17" wheels thread here:
http://ecomodder.com/forum/showthrea...els-23509.html