On the Tesla Motors Club Forum there has been some excitement over sudden drops of rated range on this thread.
During my service visit a couple of months ago, I was told by the service manager that the rated range changes depending upon your driving conditions. Several customers were getting confused about this and even a bit alarmed thinking that their battery was degrading quite fast. The service manager said something to the effect that “the software engineers were too smart for their own good.”
I haven’t paid a lot of attention to the range after charging. I rarely even look at the battery pack as it has been plentiful, so the gauge got boring. After watching this thread, I did notice my rated range was at the bottom of the reported ranges.
I am actually not that surprised that my range would be lower than normal. I do not drive much on city streets. My driving is probably 20% on highways around 40mph, 60% on freeways without much traffic, and 20% on city streets. The street up to my house also has a very steep grade. It takes a lot of energy to get to my house as you can see in the graph below.
I think having this range personalized is useful as the data is more accurate. But I would like some other kind of battery gauge stating such as a SOC, state of charge, indicator. Eventually if you want to resell your Model S and you have driven it aggressively, this personal rated range could be a false indicator.