i wouldn't worry about overheating your engine...if it doesn't over heat in the summer, you can block 95% of the grill opening and it will be cooler than in the middle of the summer....If it does, the fan will turn on and suck the cold air from under the car or anywhere else since it's much colder now...
What's your average speed on your trips? what's the average distance you travel at a time?
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