Does the car have a gas gauge?  Why would you wait until it was almost
empty?

Yeah, take it back to the dealer.

Jeff

"Rocky" ...

Rocky,
I have a Prius with a lot of miles on it now since January.  It is best to
use the gas gauge that is next to the speedometer instead of relying on
miles driven for fillup timing.

The computer is really only there to give you relative MPGs, and is not
really accurate all that much overall.  It 'counts' how much gas is
injected into the engine, relying on what the computer is telling it.
Maybe it is right.  I don't really think so, as my readings have compared
both high and low a bit upon fillups.  Even still, an MPG reading on only
one fillup is flawed as well as the amount actually filled can vary as
different pumps click off at different points and there is likely
differing top-off parameters as well.  A good MPG reading is one taken
over about 5 tankfulls, all added together.

Are you mesmerized by that screen yet?
Tomes

The Trip Meter (odometer) displays how far the car has gone, not how much
fuel it has demanded. If you short fill the tank and still expect to go 400
miles <or whatever> then you might run out of gas before you get there.

You've only had the car for 1000 miles, you should be relying on the Gas
Guage as the primary indicator of how much fuel is remaining. The gas guage
has a handy lamp on it that comes on with _about_ 3 gallons remaining in the
tank, I suggest that when the lamp comes on, you buy gas enroute to wherever
you are going until you figure out what the actual range is on your car. The
trouble with a hybrid car is that in stop n go traffic, you may not be using
any gas at all, so you can sometimes get 600 miles on a tank, and other
times get 300. There is no way to use the odometer to get a grip on when the
fill ups are going to be needed. Well, if you were driving across the
country to visit grandma, you would be able to use it, but on your daily
commute around The Big City, it might not be of much help.

I'd not be bothering the dealership with this "problem" yet. You have not
had the car anywhere near long enough to know that it rerally is a problem,
or a function of how you use the car.

The trip computer is working fine.  The gas gauge--also called the Guess
Gauge--is crap.

Don't run the car out of gas.  When it's down to a couple pips on the
gauge, get gas.

The root of the problem is the flexible bladder inside the gas tank.  It
causes you to put more or less gas in at any given time, depending on
outside air temperature.  You can't gauge your mileage based on pump
readings.  Use the trip computer, or use the energy display with its
mileage reading.

http://autos.groups.yahoo.com/group/toyota-prius/surveys?id=444783
(you may have to be a member of the group to view the poll question,
but...)
"On average, what is the difference between your mpg calculated by the
car's computer and by the gas station fill-up method?
515 responses so far:
Choices  Votes  %
computer 5mpg or more higher  20  3
computer 4mpg higher  39  7
computer 3mpg higher  71  13
computer 2mpg higher  108  20
computer 1mpg higher  12  2
both methods about the same  136  26
computer 1mpg lower  5  0
computer 2mpg lower  9  1
computer 3mpg lower  16  3
computer 4mpg lower  4  0
computer 5mpg or more lower  7  1
varies too much to give meaningful average  88  17
"

My understanding of the displayed consumption screen MPG on the Prius:
The amount of gasoline used is calculated by counting the number of
"squirts" from one of the fuel injectors (and extrapolating for the
rest of the fuel injectors to get gallons or liters, depending on
market), and the distance is found by counting the number of wheel
revolutions (to get miles or km depending on market).

As for how accurate the display is, it really depends on your
vehicle.  (see the above poll question.)

One or two tankfuls aren't really helpful, thanks to the shape-
changing bladder tank found in the North American Prius.  (Outside of
the US with the standard metal tank, they don't have the wildly
varying fillup problems.)  You really have to average over several
tankfulls.  (Especially since you cannot be sure that the gas station
pump's shutoff is going to happen at the same point all the time,
anyways...)

For example, you can view graphs of my 2001 and 2004 Prius here
(although I'm lagging in my last few fillups since I moved last year
and rarely drive them anymore):  http://www.kluge.net/~felicity/prius.php
and check the raw data link under the bar.  You really need to do a
rolling average and not rely on any one tank's data.