Personally, I also found decreased mileage in my 08 Jeep Commander gasser. I think part of it is physics. Colder more dense air needs more fuel to keep the air/fuel ratio from being too lean. Likewise, warmer air needs less fuel to keep the ratio correct. I also heard of the "big rigs" and people with turbos talking about "coking" of the oil and would let it idle to cool things down (respective term). I noticed long warm ups did not help my EcoDiesel warm up as quickly as using the remote start for as long as it takes me to walk to the vehicle (a minute maybe), then, driving it at a reduced speed without hammering it till it comes up to temperature. As far as the high idle function goes, on my 2015 RAM, it surprised me when I started it on a chilly morning, then, pushed the heat all the way to the hot setting and the idle increased. I was like "what"? Tried it again and that was the trick. Now, when I park the vehicle I push the heat to the max temp and get high idle immediately upon start up. My cousin said "you're making that up" until I proved it to him. He is a life-long Ford man and he made fun of me for buying a "Dodge" until he spent time with me in the RAM. He came away with a new attitude towards it!