'87-'88 Fieros hold 11.9 gallons.
Some gauges read wrong (emoty at 1/4 tank, etc.), but you should still be able to use most of the gas that is in there.
I have owned many '88 Fieros and I could squeze out every last drop in almost all of them. My current daily driver ('88 GT) will go until it is nearly bone dry......when I fill up it is usually 11-11.5 gallons, so when I'm empty, I'm empty!
Your best bet is to fill your tank until it is completely full and drive it until it literally runs out of gas (make sure you have a 1 gallon gas can!). This will tell you when it is empty (look at the gauge to see where it's at) and you can figure out your gas mileage too. That always helps when you are getting low......if you know your mileage you can extrapolate how much gas you should have left in the tank.