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 semi truck gasoline tanker holds 9,000 gallons, give or take. The capacity is usually controlled by the 80,000 lb gross weight limit for trucks in the US. (I can't speak for other parts of the world.) A gallon of gasoline weighs about 7 lbs, and the truck weighs quite a bit itself, so you don't see liquid hydrocarbon tanks exceed 10,000 gallons.