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.
How many gallons are in a metric ton of petroleum? 1 US gallon is 3.8 litres. The density of petroleum relative to water is about 0.85 (it varies according to the quality).