When you burn gasoline, the main result is water in the form of steam that is too hot to be visible.(steam is only visible when it is condensing and you see the liquid water droplets)When this steam hits a cold exhaust system it turns to liquid water, when the exhaust warms some you see a mixture of steam and water that is the white smoke out your exhaust. When the exhaust is fully heated up the steam comes out as all vapor and you no longer see it.