Of the vehicles that have been reported to the Plug In America Model S owner survey, 32% of the 2012 model year cars have had their drive unit replaced at least once, 27% of 2013's, and 18% of 2014's and so far none of the very few 2015's reported. As you can see, the older cars have higher numbers. Is that because Tesla has improved the newer cars or because the newer cars haven't been on the road long enough for them to see the issues yet? To answer that question, one has to analyze the data using methods from reliability engineering. That's what the folks a Green Car Reports did; see page two of their article for more details.
I've looked over their methods, studied the relevant mathematical models, and their math looks good to me.
To get a clearer picture, we need more data. If you haven't filed a recent report on your Model S, please use the
survey form to file a first report or the
update form to add current data to a previous report.