How big should my gas tank be? Car was an 85 2.5 and now has a 2.8. when I swapped fuel pumps I saw the old one was against the tank floor like it should be and the sock was fairly clean, and the inside of my gas tank looked like everything was in order... But I have run my car out of gas before to the point where it stopped, and filling it up completely was only 7.5 gallons. Ive never filled it more than that to get to full. I thought it was supposed to have a 9 gallon tank?

Your car would have a 10.2 US Gallon tank. Part of the reason you can't put the full 10.2 gallons in is because you can't empty it totally with the fuel pump before it starves, although 2.5 gallons sounds excessive.

The 4 cylinder and 6 cylinder take two different pumps. The reason for this is two very different operating pressures. However, over the four years there were several different fuel tanks and pumps including a change in size of the tank. However the aftermarket manufactures have condensed the V-6 fuel pumps for all four years to one part number. If the two or three different pumps also had different dimensions when they condense them the shorter pumps have to be the ones to use because they will physically fit into all the tanks (as opposed to the taller pumps not fitting into the shallower tanks). If this is the case (and I am guessing here) then a shorter pump in a taller tank will result in more fuel left in the tank when the pump starts to suck air. The sock on the bottom of the pump should be “squished” on the bottom of the tank. You could measure from where the sending unit sits on the tank to the bottom of the tank and then the depth of the sending unit itself. If there is anything more than 1/2 inch difference I’d say that’s probably the issue.

I was pretty positive when I replaced the fuel pump that it is all the way against the bottom... It is just pretty annoying I guess. I guess the only way to know how much it actually holds is to fill it up when it is empty and out of the car... IT was like this before I replaced the fuel pump too though, its been extremely consistent as to how much filling up the tank is. Always right around 7 gallons

It takes a special procedure to fill a Fiero tank. Fill until the pump clicks off, then squeeze it just enough to start the gas flowing again, and let it fill again until it clicks off. You'll never get the full 10 gallons in, but you'll get closer.

I’ve seen this complaint before so there is something going on here related to how much fuel is left in the tank when the pump starts getting air (you run out of gas). I’ve got an 87 GT with an 11.9 gallon tank and I consistently get 10 + gallons in it when I let it get that low (the gauge just above “E” which you shouldn’t do). I struggle to believe it’s normal to get only 7 gallons on a 10.3 gallon tank. I also can’t agree with TopNotch’s approach. You run the risk of overfilling the tank which will lead to raw fuel getting into the vapor canister which will lead to all kinds of bad things. Does the fuel gauge read that it’s not completely empty when you go to fill it and/or does it run out of gas before the gauge reads empty?

Really simple:

Fill it from 1/4 tank.

Divide number of gallons you put in, by 3.

Multiply that number by 4.

The result should be very close to the size of your tank. If it's way off, something is very wrong (perhaps your gauge is off a bit).

Nice trick Dobey. I'll have to remember that one.

Yea gauge reads zero exactly when I run out of fuel. And it reads full when the tank is full. Is there a reason why maybe I couldnt fill it all the way? The gas gauge registers full so breifly and sometimes not at all.... I just blamed it on the gauge though

I've had this issue with one of my fieros in the past, one of the reasons is because of the plastic inner structure designed to keep the gas from sloshing around too much.
another is if you either have a big dent in the bottom from improper jacking (by shop or previous owner usually) or you may have a small hairy gremlin living in your tank that created a home for itself.

does your tank have any dents in the bottom? because that will push that inner plastic structure upwards and create a space where gas can remain even when the tank runs dry.

no matter what, I hope you can figure it out.

when I got my 87 I could only get about 7 gallons in from empty ( ran out ) to full ( pump shut off ) droped the tank . I found the pick up tube had a split in it so that it was drawing in the gas from the slit and not the bottom . when the gas got bellow the split it could not suck in the gas from the bottom instead got air from the split in the tube. In efect I was out of gas eventhough there was still plenty in the tank if i remember right when i droped the tank there was still 4 gallons
something to think about anyway

Well, one way to think about it is as long as the gauge reads empty before it actually runs out it really doesn’t matter how much it takes to fill it except for the fact that you’ll have to do it a little more often. I never try to let mine get below ¼ tank anyway.