I put an experimental partial grill block on my manual shift 06 mazda 5 (
http://www.google.com/imgres?imgurl=...iw=862&bih=571 )that covered about 2/3rds of the large lower grill opening, and took a trip up north to visit relatives that I've made many times before, so I had a fairly reliable baseline of about 30 to 31 mpg on this trip. Keep in mind that the epa hwy mpg is only 27 on this car. I do run the tires at max sidewall pressure of 50 psi.
When I returned I squeezed in 15.44 gallons on 497 miles for 32.18 mpg. Thats 3% more mpg or about 16 extra miles from the tank for $1.00 worth of coroplast. The mod has already paid for itself.
I watched pretty carefully to see if the motor ran any hotter at all, and except for a bit faster warm up, I really couldn't see any difference at all. The car did seem to coast better, needing a bit less throttle, but I have no way of quantifying this result.
There may be more to gains to come by going further, covering even more grill, but I'm going to wait for really hot weather to ensure I haven't already gone too far. I don't want to have a car thats borderline overheating in hot weather.