Ok, I've got 1200 miles on it. Bought it with a full DEF tank. I've just been highway driving it and took a 6hr trip through the mountains of WV yesterday (not towing). It was working pretty hard up the hills and message came on dash that had a DEF mileage count down. It was counting down almost a mile a second. I started freaking out because at that rate, I'd be out in 15 miles! Once past some hills, message went off and the DEF levels showed 3/4 tank.
So... was going up the hills at a higher rpm causing this?
Was the truck going through a regen?
As of this morning (1350 miles), I'm down to just under half tank of DEF. Is this normal DEF usage?
The more fuel you use, the more DEF you use but the relationship is not linear. It is burned in the exhaust system constantly (though at varying rates) and not just during regen, the higher the load on the engine such as hill climbing and towing the higher the rate of DEF usage is.
When towing my boat long distance for the first time I got the same scare where the truck was warning I would run out of DEF. I can only assume that the trucks look at the rate of usage for the past XX miles and project forward to give a range, this can be misleading if you are working the truck hard for a short distance and then driving pattern changes. My solution is I always carry a spare box of DEF in the bed whenever towing now, luckily it is cheap and readily available.
Your usage rate sounds normal, in my experience (2022 refresh LM2, I have heard the LZ0s use slightly more DEF) the DEF range with a full tank while not towing is 3-4k miles.
