Is it bad to mix different size tires on a car?

"We" used to do it all the time. I CAN tell you though, that with radials, especially, you can generate some interesting handling problems. One time my boss had a then new 80?1?2? Dodge 1/2T pu, and we got a new pair on the rear. Same size as front, but different brand. That thing was just plain SCARY on a DRY freeway. It felt like if you just wiggled the wheel a "little" the rear was gonna swap ends

My Dart was "accidental." I bought these rear wheels and tires cheap, and still have not converted the front to BBP. They seem to work pretty well