tnation

I went to fill up my Lude today and i put 50 liters of premium in and the gas station clicked once and stopped me. I paid, than started to drive. My car was EMPTY to the bone (14.7 gallons to fill), like the gas gauge said it was on very empty, below the last line (hard on the fuel pump I now know.. sh!t) I was just going to do a full top to bottom gas mileage test because i've been getting astounding gas mileage as of late (like 7litres to a 100km). However, as i'm driving, the gas meter isn't changing in the car... its staying at just over half. I can't figure out why, i was thinking to myself, maybe I didn't totally fill it? so i go to another gas station and put premium in again... and gas comes pouring out the side. And again i start my car and drive and the meter says its just reading over half.. So obviously my car is full, but is it my sending unit thats quirky or what? did my driving around with zero gas overheat the fuel pump?
also, so do i have to take the tank off? is the sending unit part of the fuel pump? is the sending unit accessible by the back seat or the trunk? i know some Accord's it was accessible by the trunk.

thanks guys for all your help.