I'm a little confused on what is better for mpg gains on a car. I know for example, the 2015 Toyota Corolla LE ECO, comes standard with 15" steel wheels and claims 42 mpg. The LE ECO plus, which comes with 16-in. aerodynamically designed alloy wheels, only nets 40 mpg. The only other difference is fog lights. Total weight difference between the cars is 20 lbs. On my car, the 15 Corolla LE (non-eco), it has 16" steel wheels. Out of curiosity, would it benefit more from 15" steel, or 16" alloy?