Having studied aerodynamics I can say unequivocably that the results of your efforts would not result in any measurable improvements. That gap actually gets ‘pressurized’ and acts as if it is filled. Yes, my friend who designed the Gossamer Albatross could provide you with something that would raise your mpg by, say, 0.1 mpg but, trust me, you couldn’t afford him. ?
Easing up on yer right foot will achieve more savings.
It’s good to be a n00b - so I can aspire to be just stoopid.