from what i understand, the low fuel light is supposed to come on when you have 100 miles left in the tank, but we all know thats not always the case.

but in your case it may be spot on. so if you fill up and only put 10-12 gallons in that means you had up to 8 gallons left in your tank, and if you get 20 mpg, you have 100 to 120 miles left to travel before your tank would be empty. thus adding over 100 miles to your tank.