[MaxHedrm]

There is no reason going from 17" to 15" wheels alone would improve your FE. The key is the weight of the wheels & what tires you put on them. If you get lighter wheels (so don't go with those cheap steel wheels, or even cheap alloys for that matter) you will see an improvement in ride, handling (including braking) AND fuel efficiency (particularly in stop & go situations, less so on the highway). Low rolling resistance tires will see a decrease in ride quality and handling (including braking) but an increase in fuel efficiency.

Mario probably replaced the all season performance tires with a more economy minded LRR tire to get that improvement.