Gasoline is a very flammable and potentially explosive liquid. It belongs in your car's gas tank, not flowing, leaking or sloshing out the fill spout. If this is happening to you, determine the cause and fix the problem.
Gasoline expands as it's temperature rises. If you choose to top off your tank by attempting to pour the most gas inside as possible, expect it to expand and leak out the filler spout on warm days.
Gas tanks have a ventilation hose through which air inside can escape when you add gas to the tank. If the vent tube gets clogged, the air in the tank will slightly pressurize, then burp up and out the filler spout, usually blowing some amount of gasoline out along with the burp.
Most car gas tanks are not baffled to dampen the gasoline's propensity to swish around and potentially slosh up the filler pipe and out. If your car has this problem note the amount of fuel in the tank and the driving conditions which cause it to happen. Try to avoid the driving conditions which can produce the sloshing when the tank is at the critical level.
