[JSube]

Stay with 17s, a bit wider if you like won't matter much, and maybe your mpg will improve! Who cares! (I see this your first post, please don't take this as being flamed.)
Any weight savings on a car will result in improved mileage, because math, but it won't be significant. The benefits when you save 6 lbs or so of unsprung weight per wheel is a noticeable (to me) improvement in sharper handling and spinning up the rear tires. It's a net gain in fun.
And don't worry about breaking most lighter wheels on potholes, you're usually paying more $$ for the lightness and manufacturing processes are superior. Enkei RPF1s, Kosei, 949 Racing, Apex, O.Z. etc. use low pressure casting and some version of flow forming and are very durable. You're much more likely to break a Chinese 25 lb. gravity cast POS wheel in daily use.