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 of water are in a metric ton? 1 US gallon = 3.78541178 liters. Only (!) if you mean pure water, then 1000 litres of water as a volume weigh 1 metric tonne.