Originally Posted by losiglow

I don't see how the shutter being open or closed would result in that large of an MPG difference. The drag difference is negligible and the system (coolant) temperature wouldn't likely result in that large of a difference.

A change of 2 PSI in your tires is likely to have a more significant difference on your mileage than the shutter. If you're interested in improving mileage, inflate your tires back to the pressure they were at before the temperature drop. In my experience, going from about 60F to 30F drops the tire pressure about 4-5 PSI. Inflate them back up to 36 PSI or whatever they were at when the temp was warmer, and I'll bet you see an increase in that MPG.

Also, one of the best ways to determine if the shutter is working correctly is keeping an eye on your coolant temp gauge. If it's staying relatively steady at halfway, the shutter is likely working correctly. If it were malfunctioning, the CEL would come on AND you'd like see the temp gauge chronically too low (assuming it was staying open in the cold weather when it shouldn't).