Just to add some details: I bought my 2018 Elantra in mid-Dec. I'm in the Great Lakes area, so it is COLD here. I did notice my trip computer dropped ave/MPG to around 38 when we had that bitterly cold spell a few weeks ago. Now that it's back to normal (20-30F), MPG is back up around 43-44 mi.
My commute is highway driving, Normal mode, 65 mph. Will be interesting to see if MPG changes when temps are closer to 60s-70s. Guess once the A/C is needed, that may have a negative impact. But again, I'm very happy to be getting 5-6 miles better than was posted on the window sticker!