I know that many people recommend that you do not let your gas tank get below a quarter full. The idea being that this will help keep the sediment and schmutz that settles in the bottom of the tank from getting into your fuel system. However, I have talked to a couple of people, including my brother-in-law Jim (who is a mechanic), that say this is dead wrong. A couple of years ago, when my wife and I were visiting her family for a few days, he showed me an example using two jars and some sugar water. It went like this:
He filled both jars with sugar water and sealed them both (to cut down on evaporation). Every couple of hours, he would stop by and siphon some water from the top of each jar. When one jar got down to a quater full, he would refill it with more sugar water. The other jar he would let get pretty close to empty (but not all the way!) before he would refill it with more sugar water. The day we were leaving, he showed me the two jars. The jar that he had let get almost empty each time, looked pretty clear. There was a bit of cloudyness near the bottom of the jar and a thin layer of sugar at the very bottom. However, in the jar that he never let get below a quarter full, almost the entire jar was cloudy and there was a full half inch of sugar at the bottom of the jar.
Jim said that this same thing happens in your gas tank, albeit at a much slower rate. He said that for anyone who's going to have their car for more than four or five years, that the whole "don't go below 1/4 tank" thing is a very bad idea. Not only do you end up with more overall sediment in your tank, if you should happen to dip "too low" in the gas level after you've been driving a few years, you're almost guaranteed to get a considerable amount of crap running through your fuel system. He recommended that you should let your gas tank get pretty low before filling it, and just use fuel system cleaning additives every other oil change. According to him, that will be much better for your fuel system over the long haul.
By the way, he still drives the '78 Toyota pickup he bought when he was a teenager. 250k miles on it, and still going.
I'm wondering if anyone else has heard of this?