I have 8.5 wide rims with a 48mm offset. I would not want any less, I would bet it would rub the fender. It's very close. Less offset means the wheel sticks out farther, so a 38mm offset would put the outer lip of the rim 10mm farther out than a 48mm offset. Yes, the tires will make a difference, but if you are at 18 x 8.5, the ideal tire is 245/35/18. 225 will be stretched to make it across that rim. Unless you are going for the lowrider look.
I seriously thought about an 18 x 8.5 with a 245/35, but chose to go with 17" because I have a much better tire selection. I might still do it for the wife's car though. She'll like it.