One member here went from 15" wheels up to (much heavier!) 18" wheels and saw very little loss in the city, and a gain on the freeway. A green Scion xB, I think it was?
I think this was the thread I was trying to remember:
http://ecomodder.com/forum/showthrea...der-17440.html
I was off a bit; it was 15s up to 17s. And I misremembered the results, they showed pretty much zero change.
-soD