I have an 85 gt and from the research I've done the gas tank should hold about 10.2 gals of fuel. Mine won't. I've never put more than 6 gals in at one time. Last time I filled up the gauge read one tick under 1/4 tank and I put in 5.87 gals and I kept filling even after the gun shut off automatically.

I did the math and if I put in 6 gals to fill up from a 1/4 tank, that's 2 gals per 1/4 tank. So my tank apparently only holds at most 8 gals.

I have no clue what the problem is. I usually go 100-110 miles per tank before refueling and usually get 18-21 mpg.

Any ideas or input would be much appreciated.

My 88 says it holds 10.9 i believe it was but ive ran it to about an 1/8 of a tank and can only get 8.5-9 gallons before its full, I read that even though the tank can hold 10.9 the usable capacity is only like 9ish, but you have a huge difference compared to what im managing, not sure, good luck though

84 to 86 tanks are slightley smaller than the 87 to 88 tank.
A failing sending unit will throw the gas gauge off by a few gallions.

Your fuel pump pick-up may be off, so you don't get all the fuel out if you run out of gas. The guages are notoriously inaccurate and not believable.

That was my initial thought, that the gauge was reading wrong. My plan to test it was to run it until it hit E or got really close to it, depending on what happens as I get closer and closer to E, and see if car keeps running when the gauge reads less than E. Then I would have an idea as to whats wrong. Or eliminate a possibility. Is this a good idea or bad idea. This is the only way I can think of to see if a faulty gauge/sending unit is my problem without replacing the sending unit.

My 88 Duke Coupe ran out of gas when I first bought it with the fuel gauge showing a solid quarter of a tank. From research on this forum it seems that fuel guages....(well guages in general) not working correctly are very common. I use mileage to calculate when I should get gas and ignore the gauge. I get as good as 38 mpg on the highway with no AC. I rarely go over 70 mph. Im in Florida so the AC is on MAX mode on hot days. Its more like 23 to 25 mpg combined highway and city driving. So I always fill up around 175 miles to keep from walking again.

My 85GT (like almost all Fieros) needed to have the sender adjusted. I can run 160-180 miles to and from work and be just under a 1/4 tank mixed driving. Last summer I drove a little over 240 miles on one tank flirting with the E the last 10 miles so adjusting is worth your while when you have a chance some day.

Remember it's hard on the Fiero pump to run the tanks very dry. The fuel cools the pump and you start out with a smaller volume of fuel then other cars and it heats up faster while recirculating. The more times you run the tank low the less life your pump is going ot have.

Don't forget that the Fiero tanks are very long tanks. Unless the pickup is right on the very bottom of the tank, it's possible to have upward of 2 gallons that might not be usable. On the GT's with the larger rear wheels, this would make the residual puddle closer to the front, away from the fuel pickup screen.

The first thing I do whenever I drop a tank is to pull the sending unit and recalibrate it to a more accurate reading. You need a fine 600 grit sandpaper to get the corrosion off the contacts if it sat for a while. If not, use a t-25 to adjust the resister coil so that you get 90 ohms of resistance at full articulation and AS CLOSE to zero you can get for empty. You will not get to zero but you can get to 3 or 4 ohms. good enough. They come from the factory all over the map. I've seen them calibrated at 16 ohms to 130 ohms!

On a new to me vehicle, it is important to know when it will run out of gas. Pull the fuel line at the filter and put a hose on the end closest to the fuel tank. Jumper the fuel pump and let it pump the tank till empty while watching the fuel guage. It may never get to empty or it may get to empty with several gallons left in the tank.

2 thoughts.
1) as stated earlier, if you are running larger tires in the rear your tank may be tilted down in the front enough to hold several gallons that the float can not be IN so it will show empty or very close and you can only put in 7 or 8 gallons.

2) one of my cars has a LARGE dent in the tank from PO which probably takes up close to a gallon of space and/or could stop float from coming down as far as it should.

Mine runs dry just over the E mark. Done it twice now. Only takes 8.5 to 9 when it's completely empty.