My wife drives a 1998 Chryler Town & Country Minvan, with a 3.8L V6 and an automatic.
The test's. First test I took a roll of saran wrap and covered all of the grill opening above the bumper, as well as the lower holes in the bumper, leaving the one large opening in the bumper. My wife didn't say anything, but my son in law did ask her how she can put up with my doing this kind of stuff. Anyway, the result's. The resettable average gasoline useage meter was zeroed out and reported about 23 mpg, over the course of a roundtrip to Orange County, CA and back, to Ventura, CA.
Second test, I removed the saran wrap and placed a sheet of cardboard in the space between the radiator and the A/C condensor. I had to put two slots in the cardboard, about 2 inch's wide, by 8 inch's deep, to allow the car to not get any warmer than normal. Over the course of basically the same round trip, the mileage reported was 24 mpg.
Third test, I removed the cardboard and returned everything back to no block. Over the course of the same drive, the mileage was 20.3.
It appears to me that in this particular vehicle, the grill block is capable of increasing the mileage by about 15%, based on these test.
__________________