Quick update on the grille block. We now have 3 fillups since the grille block was installed. And average fuel economy has increased. Before the grille block, the average was about 57.7 mpg. After the grille block, it's about 60.7 mpg.
I still think some (if not most) of that is environmental variables. But I don't have the resources for a true scientific test. So I'm going to call it a win, and move on. Sometime in the future, I'll make a "grille block grille", so the car isn't driving around with foam rubber stuffed up its nose.
Also, I've decided to do the mirrors next. I got some bright orange yarn, for tuft testing. Should be fun!