I have a block heater and the grill blocked off by a huge percent. (And even at that the engine never has gotten above 186°F all winter, even climbing steep mountain passes.)
But on the other hand I have studded snow tires. I've gotten stuck at least three times and have had to put chains on. And temperatures have gotten down to -30°F quite a few times. Add to that winter fuel blends.
So overall my fuel mileage went south for the winter. Hopefully it will perk back up once things warm up enough to merit putting the all-season tires back on.
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