Not to burst your bubble(s) but....

I don't believe you can get an 18" wheel/tire combo to be the same O.D. as stock. Definately not the 19"s. I think you are looking at least 1/2" taller min.

Do you guys understand exactly how the Synergy drive works? It's a pretty intense control system.

I would be very, very, very cautious about going to larger O.D. tires, my gut feeling is you will send the control system into a wild freakin frenzy, but I could be wrong...

You can get the total diameter to be equivalent. Remember to disconnect the battery to reset the ECU's adjustments to you and it'll readjust to the new rims/tires. I'd be cautious, though, and try to avoid heavier wheels because they'll take a toll on mpg. I know a couple of prius guys who've popped 17s and even 18s on their cars with no problem

Tim, I have another question, how much do your 17s weigh?

No idea, maybe someone at Nissan could tell you...