Great articles, both of them. But I must point out FFs is from 2014. No, I don't think anything has changed; however more current info should be available.
My wife's 7 year old Mustang gets 30 highway and 22-23 around town. My Fiero got 32 going from Atlanta to Augusta when I first bought it, 20-23 around town. Look at old Pintos, they made over 30 mpg highway. Some Chevettes made 50 mpg. The Mustang is bigger, heavier, and still has an explosive Takata airbag on the passenger side. It also handles poor weather worse than the Fiero, despite all the electronic gizmos.
Mustang generally performs slightly better from 0-Speed Limit, and I do like its ABS.
Rambling along... Have you considered the environmental impact of all the employees driving to work to build new cars? Just toured Rouge River in Detroit where they make the F150s. Lots of workers. And how about the folks at the power plants who keep the line powered? They don't sleep there.
My point is there are lots of factors and cost in building new cars beyond mpg.
BTW, new base Mustang, ~$25K. Ten year old Mustang, with poorer mpg, under $10K. That's a lotta gas for $15K.