MILES per gallon ? How about GALLONS per mile !!!! :roflol:

:wtf:

I guess you never saw a Pro Mod.

no. That was my reaction on OP pictures😁

OK missed that, I too saw what was done and said the same thing :dizzy:

uhm....
Ive been part ecomodders for some time, over 10 years, and never seen any of them thought of doing something that could intentionally damage the engine long term.
yes, they block off some grill areas, but only partial to add streamlining to windflow.
as was mentioned by donbrew, look at closing gaps along the vehicle
create and install a belly pan on the underbody.
blocking off the foglight inserts is a good idea, but making them smooth is better than leaving a bulge like the pictures show.
unfortunately, there really isnt much that can be done to the rear to allow better draft release, outside of the large diffuser posted by oldblue, which I dont recommend.

I did some techniques to my old 99 ranger that enabled me to get a solid 24mph at interstate speeds (70) here in FL, when the truck was rated to only get 19mpg at 55mph. my goal was to reach 28mpg, but I decided instead to purchase something that would give me better mpg right off, and allow for family to sit comfortably ;)
on my mazda p5 I did 2 partial grill blocks, using perforated metal. 3/8 holes X 3/4 spacing. this enabled me to get upwards of 30mpg at 70mph (27mpg was best I could achieve stock using hypermiling techniques) but my temp was always in question as it tended to run between 1/2 and 3/4 on the gauge instead of just below 1/2. eventually this would lead to scorched piston walls, and damaging the seals (all the seals). so I removed the upper grill cover and satisfied myself with 28/29mpg at 70mph, but Im not done, next will be a full belly pan that will extend from nose to tail, blocking off almost the entire underbody. Ive already got it designed just have to wait until I can actually take the time to make it (I work in an aluminum shop) and fine tune it. but considering how expensive a 5x8 sheet of aluminum is (and I need more than a single sheet) it can wait. lol

its always like riding a tightrope, the best mpg you can get without causing undue damage to the engine. unless, as was mentioned, its for a short term project, then by all means, go crazy.
but definitely not something for day to day.

I always got about 32 MPG at 70 MPH; 33-34 at 65 MPH with no mods.

If you put a full belly pan on it , where does the cooling air flow out? I guess you could do some science to figure out the size and direction that is optimal.

I designed it to have grills at the bottom, facing towards the rear, think shark gills in design, 2 sets of 4, around 6" in length, on either side near the front struts, angled outwards towards the side. then a single set of 6 towards the middle, also about 10" long for each slit. spacing for the 2 sets would be 2" apart the middle one would be 3" spacing between gills.