Tesla firetraps? Numbers don't back it up

News of a third Tesla Model S battery fire Wednesday has ignited a wave of safety concern, Internet debate and investor alarm. Tesla's stock price opened today at $135.89, down from a high of $194.50 on Sept. 30.

I'll leave the battery chemistry and vehicle structure questions to experts and the stock price to Wall Street. But let me offer a little back-of-the-envelope math to give some perspective on the frequency of Tesla fires.

In its third-quarter earnings letter this week, Tesla said it has "over 19,000" Model S owners. Divide that by three fires and you get one fire per 6,333 Model S units.

According to the U.S. Department of Transportation, as of 2011, the last year for which figures are available, there were 253,108,389 registered vehicles on U.S. roads. The National Fire Protection Association estimated that in 2011 there were 187,500 car fires, killing 270 people. Doing the math gives you a car fire per 1,350 vehicles in that year -- a considerably higher rate than for the Model S.

This is a rough estimate. It assumes that the entire U.S. auto fleet has internal combustion engines (although I think the discrepancy would be trivial). Also, there aren't any dilapidated 20-year-old Model S cars in operation, while legions of petrol-powered beaters are wheezing along the roads. Finally, it would be more precise to calculate the number for fires per miles driven.

But these admittedly rough numbers strongly suggest that -- at least at this point -- fires in Tesla's electric vehicles are considerably less common than those in vehicles powered by gasoline and diesel.