I hav a 2003 mountaineer 4.0L v6 with 52K. I was getting around 16mpg this summer, I installed a KNN cold air intake and got pretty much the same. Now that winter is here, I'm hovering around 12.5 to 13 which seems much worse than what I should be getting. Air filter is clean, no MIL lamp, fuel filter changed, no wheel bearing or tire issues...I'm not sure if this is normal. I know average is around 14-16, but 12 seems pretty drastic. It has no performance issues and still has lots of power. Any suggestions anyone??

 

colder air and free flowing intake cause the engine to take longer to warm up to operating temps. until that happens, it runs rich to help the engine warm up. You will loose a little mpg this way. You can augment this by creating a divert for the intake to draw in air from under the hood until the engine is warm and then switch over to the fresh side. It may still get some drop in mpg, but won't be as drastic. I'm having to deal with this myself, but since I'm in Texas, I know the cold won't last long and I'll only have to deal with filling up more often for a short amount of time.