gas tank and front diff. question

[newkid17]

my girlfriends svx is getting crappy gas mileage (so i thought). i would take it into town, and back home(app. 12 mi. round trip) and i was using about 1/8 of a tank of gas. so i told her to run it down to E. and fill it up to see how much gas it takes, it only too 8 gallons. our tanks are 18 gallons right? so what i am thinking is either the sending unit is bad or there is gas in the float which would make it read less than whats in it. ????what do you guys think?

and i have been changing my front diff. and i went to put fluid in it last night and the dipstick was not coming out, anyone ever had this problem before? i think i am going to try and heat it up because maybe it is frozen? it has been sitting outside and its went down to around -20 last week.

Bobby