You can't optimize for two things at the same time in the /same system/.
In your example, the best volume for the stereo will change based on how fast you're going. Noise from air going around your car will require a slightly louder volume. It technically goes the other way too, I'm sure louder music will affect your mileage slightly, reducing your MPG due to increased power draw. Realistically it won't matter, but they do exist in the same system.
You can absolutely optimize for two things at the same time in different systems. The ideal speed optimizing for MPG in different cars aren't really linked and can be found independently.
In your example, the best volume for the stereo will change based on how fast you're going. Noise from air going around your car will require a slightly louder volume. It technically goes the other way too, I'm sure louder music will affect your mileage slightly, reducing your MPG due to increased power draw. Realistically it won't matter, but they do exist in the same system.
You can absolutely optimize for two things at the same time in different systems. The ideal speed optimizing for MPG in different cars aren't really linked and can be found independently.