i have a 1985 mustang and and wanted to know what the widest wheels are that can be put on this car with either 18 or 19 inch rims  

I'm not sure about 19s, mostly because i've never heard of anyone putting them on a Fox Mustang. I've got 18X9s on the front and 18X10s on the rear of my 1993. The front 9s will require a steering limiter otherwise you'll end up hitting the control arm. Tire size is even more important as you'll want a sidewall that curves inward, not one that is straight. A 255/35/18 on the front and 265/35/18 rear should work well. The other issue you might run into is the front lower fender extensions. Ford revised these in '91 to accomodate the larger 16-inch wheels, and depending on what tire size you go with, you may run into a slight rubbing issue there. As for the 19s, stick with an 8-inch rim and you should be OK as long as you find some rubber bands for tires. I've seen 20s on a Fox Mustang so its certainly possible.

Steve