From the way I understand it...the fleet CAFE ratings are based on standards from the late 1970s and haven't been adjusted, thus the 42 MPG requirement is actually 25 or so MPG based on the "real" world MPG that the current EPA testing uses.
Before anyone asks the question. The will by no vehicle scheduling for any Ford or Lincoln models for next week... the week of 11/19/2019 which is Thanksgiving week in the USA. After this week's scheduling on Thursday 11/15/2018, scheduling will resume the week of 11/26/2018 with scheduling done on Thursday 11/29/2018.
From the latest (11/09/2018) FDNB (Fleet Distribution News Bulletin) here's the latest 2019/2020MY Mustang production schedule.
05/03/2019 - 2019MY Fleet Final Order Due Date
05/16/2019 - 2019MY Last Day to Spec Change
08/08/2019 - 2019MY Balance Out Date
05/14/2019 - 2020MY Order Bank Open Date
06/13/2019 - 2020MY Scheduling Begins
08/12/2019 - 2020MY Job #1 Date
Fuel prices are not the half of it. Factor in purchase price, poor fuel economy, expensive repairs, and reduced longevity these new diesel are rapidly loosing their ownership cost advantages over spark-ignition engines. 7X is a good move, I can see it eventually outselling the Powerstroke if trends continue.