For the record, I don't think anyone guessed the "secret R&D" spending was another model. This is pretty great no matter what it is... I am betting the "other thing" is the Model X reveal, right? right? or just the driver assist stuff which hasn't really seen public release yet.
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Engadget has immediately joined into the speculation suggesting it might be a smaller SUV that will join the Model 3 on release... this at least makes sense since we know they have talked about it in the past in passing. I think I like this idea and fits with things that has been suggested previously here.
Elon Musk teases Tesla 'D' unveiling on October 9th
Sorry forgot to post the article.