I own a 2012 Model S P85 Signature that I purchased used in Sep 2016. It had approximately 60K when I purchased it. I love the car. I just recently crossed 80K. Here is my observation and concern with battery degradation. Since 60K my car has fully charged to 240-245 miles and 90% at 220 miles. I recently took two road trips two weeks apart. The first was to Shreveport Louisiana from Chattanooga TN. On this trip I experienced the 240 miles range. Two weeks later right after crossing 80K on the odometer, I travelled to Gulf Shores Alabama. On this trip I could only achieve 230 miles for 100% charge and 207 miles for a 90% charge. I understand that my battery will degrade, but I don't expect it to happen in 10 mile increments. My hypothesis is that Tesla is limiting charge based on mileage. There is no other explanation I can give for this step change over a two week period. The weather conditions were similar, and I used multiple superchargers on both trips. I understand if Tesla wants to program limits into my Tesla to preserve performance of the battery. It would be nice if they would tell me what those are so I can know what to expect.