Interesting that most of you are finding such large temp diffs between ambient and internal fermentation. On ales, I agree, I find such differences of four degrees or more. But on the two lagers I monitored this winter, the difference was only two or three degrees, max. This was with two digital thermometers that were calibrated to a standard at the fermentation temp, of 48 - 52 depending.
This was using a plastic six gallon bucket in a fridge, controlled by a JC A419.
Edit: Looked back through my notes, and it was not ambient air temperature that I was measuring. Rather, the temp probe was taped to the bucket exterior and covered with a double layer of bubble wrap. So that probe measured only 2 or 3 degrees difference from the exterior ferment temperature. I generally do not measure the ambient air temp inside the fridge as it fluctuates so much. I keep the JC probe taped to the side and under bubble wrap now.