Since I bought my VW last fall I have not done much to it. Changed tires, put a grill block on it. The weather had gotten colder mileage was still good but had dropped off to about 40 mpg.
Last week we had some weather approaching 70 degrees and I decided to take my last piece of election sign and make a hood splitter. I had also pulled the bottom of my grill cover out and attached it to the front of the bumper.
I carved it into a D shape and heat molded it with a propane torch. Right now it is duct taped on.
Earlier this week I was driving in the rain and really noticed a change in the pattern of the drops on the windshield. The yellow lines mimic the pattern they made on the windshield.
I can't say specifically the effect on the mileage. It may be good. I have 280 miles and it is just at 3/4 of a tank. Which is further than usual but not quantifiable.
Any comments on the air flow toward the center. None of the rain drops went past the center line of the windshield.
I may make some brackets and raise the back of the hood.