Fuel Sending Units

[fixkick]

I have a 1995 4 door Sidekick 1.6l 16 valve engine, 4wd, manual transmission. 

I’m having some issues with getting fuel level to read correctly. The needle works, but when I fill up it doesn’t go all the way to full. It will go to around 3/4 or 7/8’s. Now if I stand there and very slowly pump about 2 more gallons of gas into the tank (like a third of a gallon at a time) I can get it closer, but its dang annoying and something I don’t love doing. 

A little backstory on the vehicle. It originally had the bigger tank, but several years ago got some rust in there and had to be replaced. The shop that replaced it said they couldn’t find the bigger tank so we put the smaller one in there. I do not know if they replaced the sending unit at this time or not. Then more recently while I’ve been restoring it, I found the bigger tank from Japan, so I ordered it and thats what’s on the car now. The sending unit was replaced at the same time, but it’s reading like I described above.

I’m wondering a few things:

1) I’ve seen different part numbers for the sending units. Does that affect how they read, or is that more just about fitment in the tank? Like, if you put the sending unit for the smaller tank in the big one would it read correctly? And vice versa, if you put the sending unit for the bigger tank in the smaller tank would it read correctly? 

2) What else could cause this, and what could I do to get my needle to go all the way to full upon fill up?

I have searched up, down, and all around for info about this and can’t find exactly what I’m looking for. So any information about this is more than welcome. 

Thank you!!

not sure on the details there,?

• the needle never goes full (max)?
  
• the tank not full, ?
  
• or both.?
  

I will guess #1
if the gas station nozzle clicks off too soon, is this it?  that means 1 of 4 things
first off no tank on any car fills 100% ever, (or on a hot day the fuel expands and the gas leaks out vents)
1: the pump nozzle at the station is bad, use  a different gas station to prove that.
2: the cars filler nozzle  has bad vents at this point or is missing parts. so air can not reach that point .
3: any or all EVAP tank vents blocked, hoses pinched, etc.
4: or is normal (calif nozzles are  a PITA)

is this  a STATION fillup California style nozzle, with huge bellows and spring,  if yes that is even more tricky to get to work on old cars.
a non calif. nozzle is just a pipe. just metal. and lets air sneak around the pipe making easy fills on the tank.<< never fails  to work mostly.

I drove motor cycles for years and calif nozzle all fail hard as my MC have no vents to all allow calif nozzles to work unless I hand forced the bellows back so air can flow and not trip the filler valve off line.closed. I;d go out of my way to find stations with normal filler nozzles on all my bikes, (had 6 then)

the FSM covers the gauge test, we simply short out the wire to the thank fill pot , the tank filler pot is  huge variable resistor (wire wound) and goes super low resistance
full. (at filled fully up)  shorting it to ground is the FSM steps on testing the GAUGE, easy as pie.

here is the spec on 96 tank.