FMM will run until you use it all, OR, until you add enough fresh gas to change the average age of the entire tank. So, if FMM started while the tank has 8 gallons in it. You ran out 4 gallons then put 4 fresh gallons in, you would have 8 gallons whose average age has been changed to 6 months old. Now FMM will start again in only 6 months. The only way to get the full 12 month window back is to let FMM use all of the old gas, then only refill with 2-3 gallons of fresh.Can someone please explain the relationship between the amount of fuel in the tank and FMM running? Shouldn't you be forced to use up old fuel regardless of how much is in there?