I just finished my second tank on my 2019 GT Reserve.
I live in a hilly, winding rural area with little stop & go (but lots of accelerating/decelerating through the curves), and mostly 45MPH or over speed limits.
The first tank was 23.4 by the computer. I did not manually calculate it because I had no idea how the dealer topped it (or didn't).
My second tank was 25.3 manually calculated and 24.8 per the computer. This tank had 100 highway (70MPH) miles on it.
I was returning home from a trip today and noticed that I was near the 600 mile break-in threshold, so I drove around to hit it. I then did what I've been dying to do since I got the car. Let's just say that the turbo is amazing, and while manual shift mode ain't the same as a stick shift, it makes these back roads a lot more fun to drive. That being said, I'm not certain what kind of mileage I'm gonna get out of Tank #3.