My early experience with 17" rims and non-LRR tires is that there is some MPG difference, which is almost negligible in city driving, but more pronounced at highway speeds -- 65-75 MPH.

On the highway, when I drive 60-65 MPH, I can still get 50+ MPG. Going over 65 MPH, my MPG seems to drop significantly. While a severe drop in MPG at speeds nearing 70 MPH is true with any HCH I, it seems a bit worse with my current setup.

I would recommend getting the tires and rims that you want for your automobile. I believe that what is important is relative MPG; that is, what MPG are you getting compared to what you would be getting in a similarly-equipped non-hybrid equivalent. I.e. getting new tires and rims will have a similar MPG effect on the HCH as it does on a non-hybrid civic, and so your relative MPG differntial will remain consistent. You get the idea.

Anyway, like Tim said, we will know more after we have more miles and friendly weather under our belts.