On some the centre hub is a bit different.
Yup...
5x100 is the bolt pattern, but for basic fit the center bore of the replacement rim must be equal to or greater in diameter than the Fiero's hub pilot. It's not rocket science; pull a wheel and measure the diameter of the pilot & compare to the diameter of the rim's center bore if you're uncertain.
Whether or not you need adapter rings when using rims with a larger than OEM bore is debatable. By design the Fiero uses the hub pilot to center the rim, but a variety of brake upgrades render the pilots on the front hubs unuseable. Long story short, I have yet to hear of any significant complaints from people running with a wheel and/or brake configuration which no longer uses the pilot/bore fit to center the rim.
Almost last, but not least you have to consider the width of the rim, and its offset. This more or less dictates how much rim will be "in the wheel well" and whether or not it will interfere with suspension components. Offset is the distance (usually expressed in mm) of how far the rim's mounting face is from the center of the rim. For rims of equal width, a rim with a greater positive offset will have more of itself positioned in toward the center of the car. Sounds confusing at first, but sketching it out on paper usually shines the light on it for most people IMO.
Finally, rim diameter, width, and offset dictates what tire sizes you can run.
Are you wishing that I hadn't started typing yet?
These may help:
I found that once I had how offsets & rim widths work together sorted out in my head, that the above links helped determine whether or not an "unknown" rim/tire combo would fit.
If you have more questions, shoot... Alternatively if you can make it to the GTG on Saturday, I or someone else can give you a hand. There's only so much you can do with a keyboard.
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