Remember the 5.0 liter Mustangs from the late 80's that churned out a whopping 200 or so HP.
Now they have a 600 HP Mustang that probably gets about the same mileage and is superior in every way. Imagine what they could have done if they were actually *trying* (or even forced) to improve gas mileage.
Improved mileage doesn't ruin drivability or fun and recent history has proven it. If customers demand it manufacturers will be forced to deliver.
And as noted its an average and not every car will have to get 54 MPG. And there's no saying that in 2019 they won't change it to 50 or 45 MPG because its just more realistic. If you set the target at 35 the automakers will say they can only do 31. Set the target high and negotiate from there as we get closer to the date.
2012 135 Convertible
2010 Lexus RX350