I tried some MPG tests between ECO, Sport, and just Normal. This was without changing how I drive at all. Which meant that in ECO mode to get going when needed I had to give it more because it's slower. While the reverse was true in Sport mode and I could give it less for the same feel. I did 1 full tank, leaving ECO on the entire time, setting Sort as soon as I got in the car each time. All 3 tests were done before the car even had 5,000 miles on it, probably less than 3,000 even.
This is what I found for my style of driving.
ECO reported the worst MPG of the three. The reported number and the math when refilling the tank matched.
Sport reported a number much better than ECO. However, the math said the report was incorrect and and the actual math was the same as for ECO.
Normal reported the best, and the math matched.
I did it 3 tanks in a row, with about the same mix of commuting, city, highway and traffic between all three.
Based on this, for my style of driving, I've just been keeping both off in normal mode. The math and the report seem to match and I'm getting 25 MPG overall. On a log trip, all highway not stop and go, I got 30 MPG.
Assuming the report is correct then, I do best when using the cruise control on the highway, especially in moderate traffic. It'll report about 27 after my normal commute of about 20 miles country roads and 20 miles highway. Less if it's really stop and go on the highway instead of just heavy or light traffic.
I'm guessing if I changed the way I drive to match the mode more, I could probably do better in ECO and even worse in Sport. Your experience may differ based on how you drive. My goal was to see for "how I normally drive without thinking about it" which mode was best for that style.