Gasoline has a very narrow flammable range of about 0.8 to 6% gasoline vapor in air. In other words, the vapor-air mixture must be exactly as specified or the gas
will not burn, let alone explode! Note that we say vapor. Liquid gasoline must change into vapor before it can burn (although this is no huge problem since it easily
vaporizes).
For a car to explode during impact the tank must catastrophically rupture and spew a fine mist of gasoline over a large area so it can vaporize and mix with air in
exactly the right proportions. The mixture must then find a source of ignition. Automobile gas tanks are built to withstand a considerable impact force and are
usually located in a protected area between the beams of a car’s frame. Common ignition sources in the car’s engine are generally at the other end of the vehicle.
From this funny and informative site:
http://www.intuitor.com/moviephysics/
True enough. I saw a guy snuff a lit cigarette in a cup of gasoline. He was intentionally doing this to show just what you said but I think it is a stupid thing to try in general (so in other words don’t try this at home kids). It is quite possible for gas to have evaporated at the surface and that could ignite as the cigarette goes for the liquid gas beneath.
Caught a 5 gal can of gas on fire once. As the plastic can melted, the gas ran out and increased the fire many fold. I just kept sweeping it away for the building with the garden hose and let it burn while hopping that it would keep burning and not allow to much vapor to get away in an unburned state.
The firestorm looking tornado climbing up about 50 feet was a much harder thing to control and scorched the building a bit.
To keep the fire away from the building I needed to be between the building and the fire and that was only about 10 feet so I got a bit warm.
Got my attention, that’s for sure.
*:: no, I did not say I was smart, why do you ask? 
:: *
The air/fuel ratio is too rich to burn, as long as the cap is on and there’s some liquid gas in the tank.
What? No mention of the Ford Pinto?