My gas gauge fluctuates all over the place also.  I have
'81 'falia.  Around 90 miles it's at the half mark.  I go
about 250 before filling up.  So it seems like the half mark
isn't really consistent w/ the tank being half-full (or half-
empty).  So I reset my trip odo at each fillup and make sure
I fill back up around 250.  (usually takes ~ 14 gals.).

-Scott.
'81 'falia