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.
So depending on the average fuel economy (MPG), a smaller car may have a 12 gallon tank, whereas a mid-size car is likely 16, and a larger SUV might be 20+. On average, I would say 16 gallons.