Originally Posted by Transponder
Update: Blocking the lower grill opening with foam did not have a noticeable difference on millage in driving below 32°F. ....
Nor would one expect it to...
Grill blocks are intended to reduce aerodynamic drag. They are not very effective unless applied to all grill openings. Blocking the lower grill alone could, in fact, increase drag depending on how Hyundai manages air flow behind the grill.
Note that Hyundai claims a Cd of 0.24, which is well into the range of diminishing returns. Not much to fix here, if Hyundai's aero data is to be believed. If grill blocks reduce drag, the benefit will be greater as vehicle speed increases. Thus the benefit will depend how fast you drive.
Warm-up issues have a similar dependency, but this time on trip length; short trips will use a lot of fuel if the ICE never reaches operating temperature (or you set the heat to 84F, as my wife prefers).
For those concerned about grill blocks causing engine overheating, as long as you're not climbing long grades at high ambient temperatures, there's no problem. If you're involved enough with your car to make grill blocks, you have the required attention to the Temp gauge.