The New York classic Japanese mission stood out as a tough one. But the difficulty didn't go up evenly as I went through the list. Some were almost freebies, others not so much.
I'm down to a 16.x second difference in the last mission. Yay for OCD!
Crud. Just checked, and the Prius only shows instant FE on the display, so not much good for doing a MPG challenge.
No, I was wrong. We can do this.
On the right of the Prius display, there's fuel used (I think). In metric it shows "cc". In imperial it shows "gal.".
So we just need to come up with a formula that rewards a combination of lowest time & lowest total fuel use. Easiest might be to multiply time against fuel used.