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.
As stated above, one US gallon consists of about 3.785 liters. It means that to convert metric liters to US liquid gallons, you should multiply liters by 0,264. If you want to know how many liters are in some number of gallons, you need to multiply gallons by 3.785.