Will 15" wheels fit on a 85 GT? Will there be a big difference in how it drives or performance?

BIG difference, No. It will LOOK better. depends on your tires

Since you can't get good tires in the original 215/60R14 14" right now, it will improve handling if you get good tires.

I have a 86 SE V6 auto. This is esentially the same as an 85 Fiero GT auto, IIRC. I am running 215/60R15 Falken ZE912.
Using GPS mileage vs odmeter, I get a 6.5% difference. That's about 75mph when spedo reads 70mph. BTW, the Falkens I have on my car are much better than the Khumo Ecsta 228/40/17 tires that i had on the car, especially in the rain.

quote Originally posted by RWDPLZ: Since you can't get good tires in the original 215/60R14 14" right now, it will improve handling if you get good tires.

What makes a tire 'good'? I've been using 225/60 R14 BF Goodrich Radial TA tires for years - they seem ok to me...

quote Originally posted by David Hambleton: What makes a tire 'good'? I've been using 225/60 R14 BF Goodrich Radial TA tires for years - they seem ok to me...

The Radial TA is sold as a performance all-season tire. But, the tires don't grip well in the dry, and absolutely suck in the snow. I'd say this makes it a not-so-good tire. Looking at the tread pattern, it's obvious the design is about as old as the Fiero.

Well, ok; my snowy weather performance requirement is that the tires hold air to keep the car up off the barn floor.