Fuel Guage ??

Had a simular problem with my duster when bought when full it read empty and when empty it read full.
check ohms all read ok turnsed out guage (aftermarket) was to for for a GM vehicle
found gauges read as follows
GM 0 ohms when empty 90 when ful
ford / Chrysler 73 empty 10 full
AMC / S/W 240 empty 33 full

once i had this i was able to narrow it down to the type of guage hope it helps