Fit or work right? I wouldn't put anything more than a 225 on the 6" rims but you can get 235's on there and it's unlikely you'll pop a bead but they may wear out the sides or centers. I try to use the right tire for the rim +/- one thread width size.
Fiero straight line stability is very sensitive to tire combo's and you just have to be careful about what you throw on there. After fighting little gremlins in the tire wear and handling, I keep the rim and tire diameter the same. That means 215's on a 6" rim.
Terry
[This message has been edited by TK (edited 12-17-2002).]