Around the city grill blocking doesn't do much but at freeway speeds there can be marked improvements for most, as much as 5 mpg on some vehicles depending on speeds traveled. One of the things that I've noticed from doing a complete block of the center V section of the rad grill is that my windshield stays a lot cleaner during the summer bug season.

I guess I'd have to take off the grill blocking if I was to do some trailer or boat towing.
YMMV
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