About 174,000 vehicle fires were reported in the United States in 2015, the most recent year for which statistics are available from the National Fire Protection Association. Virtually all of those fires involved gasoline powered cars. That works out to about one every three minutes.
Tesla claims that gasoline powered cars are about 11 times more likely to catch fire than a Tesla. It says the best comparison is fires per 1 billion miles driven. It says the 300,000 Teslas on the road have been driven a total of 7.5 billion miles, and about 40 fires have been reported. That works out to five fires for every billion miles traveled, compared to a rate of 55 fires per billion miles traveled in gasoline cars.