Question for the GM people. I currently have a 2013 3500 with a 3.5" lift. The truck has 20x9" MKW rims with 305/55/20 duratracs and they are too big. I can't fit mud flaps on and it drives me nuts. I think the rims have a -12mm offset. If I get rims with a +18 offset, will they fit with the same rubber or will I have to downsize the tires too? I bought the truck with this set up. It looks great but I like functionality over looks.