widest tire on 6 inch steelie?

First thing you`ll probably want to do is to measure the inside dimension of the rim. This way you`ll know exactly what you`re working with. I`m famous for always using an outside dimension when describing a rim width. I always just try to keep the tread width no more than two inches larger than the inside measurement of the rim. Too large of a tire will begin to roll the tread over anyway, resulting in uneven wear and defeating the purpose of having a large tire in the first place.