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.
The Canadian government defines one gallon as consisting of exactly 160 Imperial ounces, and there are approximately 1.041 Imperial ounces in a U.S. fluid ounce.   The U.S. liquid gallon consists of 128 U.S. fluid ounces, and there are approximately 1.201 U.S. liquid gallons in an Imperial gallon.