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.
I would think that it would be proportionally very similar. I think the savings seem to come from less airflow through the radiator system and then on under the car. If that's the case, the lowered drag would be similar, regardless of the actual engine used.
I have been meaning to try these tests out on my wife's van, for quite a while, but unless I'm driving, I didn't want to have done something to bugger up her car and then have her encounter a difficulty or anything. Additionally, although she didn't say anything to me, directly about the saran wrap, she did tell my son in law that if it got lost and fell off, somehow?, that she wouldn't have noticed what happened. wink, wink
It will certainly be worthwhile to do more testing, when the opportunity presents itself. The numbers I reported on are strictly off of the factory mileage meter, so the only thing which is of relevance is the relative numbers, although my mileage in the past has had a pretty high degree of correlation, previously.
As far as the grill block, I have run them on two different vehicles, so far and while my results do vary, the variations are all in a positive direction. I am pretty comfortable that it shows a improvement, but being the fiddler I am, I am constantly fiddling to try something else, so it may be a while before I get concrete figures.
If you look at the gaslog on my car, which is an 89 Honda Wagovan, the averages line is on a slight but steady rise. The majority of my improvement in the last month has been from a variety of grill block's, as is in my notes, their.
I think I'll do an sheet metal aluminum w/ pop rivets or thin wood w/ carrage bolts and frame arrangement grille block this weekend.... My uncle, that I am living with in Houston, has all sorts of cool wood shop equipment.