Driving in snowy conditions this morning, I put my truck into 4WD Auto.

I'm told this will reduce mpg due to extra drag.

Has anyone out there any numbers as to just how much mpg would be reduced at modest highway speeds; say 55 to 60mph?

 

Hard to tell as I have never left it in for a full tank. I would imagine maybe 1-1.5 mpg. Only way to truly tell is try it both ways and check it.