Roughly speaking... burning a gallon of gas will produce around a gallon of water as byproduct.
In terms of chemistry: one gallon of gasoline produces about 217.5 moles of water. Each mole of water has a weight of 18 grams. So the burning of one gallon of gasoline produces 3,914.6 grams of water. This is equal to 8.6 pounds of water, which has a volume of 1.033 gallons.