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 has 3.785 litres, an imperial gallon has 4.5 litres therefore the difference between the two is .715 of a litre. There are 160 fluid ounces in a British gallon.   There are 128 fluid ounces in a US gallon.     Therefore, there are (approximately) 4.6 litres in a British gallon; and   (approximately) 3.7 liters (sic) in a US gallon.