I watched a video on YouTube, where the speaker showed how the ICE has essentially plateaued in terms of efficiency with regards to emissions and mileage. I'm sure they can squeak a bit more mileage, but it's not anywhere near as dramatic as say, 15 years ago.
I was just thinking of all of the vehicle's I've had over the years the mopars were the worst on fuel except for the 1969 valiant and 1973 duster of which were both 6 cylinder cars