I have a set of 20x9 wheels with a -12 offset 4.5 backspacing. The tires are 285-55-20. Will they fit? I don't mind trimming plastic, but no metal. I plan on cranking up the bars if they will fit. I heard of a guy running them. Just don't know if it was a new bs.

Put them on and see....should take less than 30 minutes to find out

they'll rub on the metal on the back of the fender well a bit.

My wheels are 5" BS and 0 offset and I had to do some "massaging" to get them to fit. I'm running 305/55/20's though. Since your 285/55/20's would be a little narrower you would prolly be in the same boat as I was since your wheels are -12 offset and 4.5 BS. They should fit if you really crank up the bars and are willing to bend/cut some inner fender. If you're not wanting to cut/bend any metal then they aren't going to fit with those tires.

Another thing, is your truck a GMC or a Chevy? They are the "same" but I think the front end's are different and they clear wheels tires slightly different. Fill in your signature with at least the year/make/model of the truck so people can help you easier if you need it.

I've got 285/55/20's on a -9 offset and had to trim metal, my buddy put put 305/55/20's on my same wheels and did about the same amount of trimming as me but he put a Cognito leviling kit, and I just barly cranked my bars