Toyota 2003 Prius Owner's Manual page 163

44.Fuel gauge
24p001a
The gauge indicates the approximate
quantity of fuel remaining in the tank
when the ignition switch is on.
Nearly full–Indicator at "F"
Nearly empty–Indicator at "E"
It is a good idea to keep the tank over
1/4 full.
When you refuel on the slope, the indica-
tor may not show the correct level.
When you refuel less than about 11.4 L
(3.0 gal., 2.5 lmp. gal.), the fuel indicator
may not change.
After the auxiliary battery is reconnected,
one segment will flash for a while.
88
Depending on the ambient temperature,
the fuel quantity is less than 45 L (11.9
gal., 9.9 lmp. gal.) even at "F". However,
this does not affect the fuel consumption
and the remaining fuel ratio indicated on
the gauge. (For details, see "Fuel" on
page 148 in Section 3.)
Low fuel level indicator light
24p002a
Blink
If the fuel level approaches "E" or the low
fuel level indicator light blinks, fill the fuel
tank as soon as possible.