Um aside from the extra weight of the 4x4 system there are no extra moving parts when I am in 2 wd. The front axle is manual hubs which are set to free spin when not needed and the transfer case is disengaged when in 2 wd. Grid heater is wired to a push button for start up. So it does not automatically come on when the air temp is cold such as it would in the factory set up. So yes that could have some effect when the air is really cold, but 10 mpg? I am proactively going to change the injectors this truck goes to the yukon and back so the last thing I need is for a breakdown up in the middle of nowhere.