Hi guys, I have 18 x 10 volk Sf challange wheels in the back. and I'm using 265/35z/18. I want to know what is the widest tire I can use 285/30z/18? will that fit?

Cant say if you will have enough clearance for that size wheel/tire combi. ? 285's will fit that 10" wheel as will any tire between 255 and 305. To keep your speedometer reading near accurate (assuming stock tire was 225x45) you should look at 285x35 instead of 285x30. Google; tire size calculator for more in depth info. eg, http://www.1010tires.com/TireSizeCalculator.asp

What's the offset you're running in the rear? I know you can run a 18x9.5 with a 45 offset to 275/35/18s.

i've seen guys on the forum run a 275 rear tire on a +37 rim so essentially since the 285 is only 5mm wider on each side, you could run a +42 or higher rim with no problems.