I changed my rims from stock 15" to 16" ,prior to 2008 CNY. So far, ,my FC has not gone worst.
In fact, i think its didnt affect FC at all. My usual FC is ard 10.6 to 10.8km/L.
Now with the KPE, I am getting between 10.8 to 11.6km/L.
My rim is 205/50/16. So FC wont be affected.