The classic way to convey the error that exists in a measurement is a formula that shows the obtained measurement (X) as a sum of a true score plus some error. One version of this formula is: X = T + e. With this formula in mind, obtaining more precise measures depends on reducing error.
  Mega is the prefix meaning one million, therefore there are one million (1,000,000) gallons in a mega gallon.
A US gallon is a unit of volume in US Customary Units. The symbol for US gallon is US gal. There are 0.26417205 US gallons in a liter. A liter is a unit of volume in the Metric System. The symbol for liter is L. There are 3.785411784 liters in a US gallon. The International spelling is litre.