Care to share your calculations? They're quite the reverse of my back-of-the-envelope, which rounds to an egregious degree, and doesn't take into account the subset on public highways or not:

150,000 fires / 250mm total cars in america: ~ .0006

3 fires / 20,000 Teslas (their expectation for this year: ~.00015 (4x fewer fires)

I'm not sure whether we should care that gas cars catch fire in parking lots rather than highways... it seems like a huge concern regardless.