I don't think it's a dirt problem; the dirt and the pickup are on the
bottom anyway, full or empty.  It would have to be floating dirt...

However, it's good advice.  Injection systems require high fuel
pressures, so the fuel pump is usually in the tank, electrically
powered, pushing fuel.  The pump relies on some amount of residual
fuel to provide cooling.  Running tank dry will likely not lead to
immediate pump failure, but making a habit of it may lead to that.

$400 repair bill for dropping tank to replace pump...

I taught my kids to treat the 1/4 mark as time to refuel;  both to
spare the pump and to avoid running out of fuel in wrong neighborhood.

Pete

If you were trying to "game" an MPG number in a road test, you'd want
to run as light as possible in every way.  I think the US EPA doesn't
even bother controlling for this one, though, when testing cars for
their gas mileage -- probably because it is too small a concern
compared to the many other factor.  There are a host of these -- just
some of them would include vehicle weight, aerodynamics, driving
cycle, cold vs. warm start, ambient conditions, engine efficiency,
driveline efficiency, and even the surprisingly varying composition of
the gasoline itself.  (A car's "curb weight" assumes a full tank of
gas and might be an unmentioned additional condition for the start of
tests. )

Somewhere on http://www.epa.gov/fueleconomy/regulations.htm are lists
and discussion of the things  they *are* concerned with.

Certainly the weight of the gasoline wouldn't be one of the big things
(and again, weight isn't the only thing).  Say a 15 US gallon tank
holds a hundred pounds of gasoline -- a lot if you're draining it out
so as to drop the tank to replace a fuel pump (speak of the devil...)
but a thirtieth to a fortieth the curb weight of the typical American
passenger car.

I don't even see too much discussion of it among "hypermilers" though
there is some -- probably among those who are  extreme enough to also,
say, leave the jack and the spare tire at home to shave a few tens of
pounds from the all-up weight of the car.

Another bit for any calculations you want to do -- the effect of
having to gas up more or less often, which requires not only stopping,
but broaching what is, to first order, a sealed system these days, and
exposing the vapors to the outside air at least briefly.

There is often an amazing amount of ook down there, especially in an
older car.  Newer cars in general,  and newer ones with polymer rather
than metal tanks in particular, would be less vulnerable -- however,
they also tend to be fuel injected, so anything that *does* happen can
injure or kill a pump that costs a few hundreds of dollars and is a
pain to get at.

--Joe

Indeed; kinda like my comment that you might want with the minimum gas
if trying to "game" the gas mileage of a single run.   (Your reason is
probably a lot more exciting  for the spectators, though!)   But
getting the best MPG  per se (never mind other factors in prudent and
pleasant driving) on a long term average is different than making the
car as light as possible for a single trip's MPG or a pass at the
standing quarter mile.

Another poster mentioned that

Absolutely.  The little red light, or even a needle in the orange
zone, doesn't mean "this is your normal hint to fill up soon" -- it
means "you have made a mistake in planning or attentiveness, but the
right actions taken promptly might still  allow you to avoid the
consequences."

It's also good to know the accuracy and linearity and reproducibility
of the gas gauge indications.  Simple concepts underneath those fancy
words,   easily determined (knowing the tank capacity) just  by
keeping an eye on how much gas is required in order to fill up from
various points. In some cars I've had, filling up, say, a 20 gallon
tank that reads 1/4 full takes 15 gallons, every time.     In other
cars, the gauge really should be labeled "Amusement device only.  Not
intended for gambling" -- either because an indicated eighth of a tank
translates into a couple thimblefuls of accessible gas, or because the
gauge (and/or its sender) has a different version of the truth at
different times.

The "wrong neighborhood" can also be the lack of a neighborhood -- and
anything else -- for miles and miles!    Additional  good   bits of
knowledge to have in your head on a road trip are how far to the
previous and next towns; how much fuel you've got;  how far that fuel
will get you (which can vary with seasonal gas formulation, weight in
the vehicle, and the outside environment); and what speed would get
the most out of it if you didn't like the answers to the earlier
questions.

(Not liking the answers suggests that you might uncover a 6 P's
problem* as you analyze the trip later on, but for now, the mistake is
in the past and the point is to figure out how to recoup...)

If wishes were horses, then beggars would ride, and the guy with the
shovel would have fertilize'.

As you know and I know and most of the people here know, but he
doesn't seem to know,  he might be able to play around the margins of
the Nova's equipment and state of tune and his driving style and milk
another couple mpg out of it, but the only way he's going to get big
improvements is to relegate it to weekend fun and pick up something
quite a bit different as his daily...

Cheers,
--Joe

* "Six P's:   Proper Planning Prevents Piss-Poor Performance.
Favorite saying of a well organized friend.

           Five years ago I noticed the increasing number of huge
vehicles on the road. Being old enough to have lived through the shock
of 1973 when OPEC turned off the taps to raise oil from $2 per barrel
to $40, and seeing what happened to the owners of huge cars, and
knowing that many large vehicles on the road means more demand, and
that escalating demand usually means rising prices, we bought a small
car. Good thing we did. I can understand the young buying big stuff
but can't understand the short memories of folks my age.
      Not only are those big pigs hard on fuel, they depreciate
enormously in times of high fuel prices. Think back to Chrysler's
decline when they failed to start making smaller cars in the early
'70s and persisted in building New Yorkers and Imperials, cars that
got 10 mpg. It nearly broke them.

    Dan