I have 285/40/18's on the back of my supercharged 2014 GT. I want to go wider with NT05R's and my only option is 305/40/18's. Has anyone put this size on the back of their car and what size rim did you need to use?

 

I'm more than sure that people are running that tire. For textbook proper fitment that will require a 10" wide wheel.

 

305/35-20, 10.5" rim.............( car is not lowered )
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My car IS lowered, do you think there will be clearance issues?

 

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Maybe depending on your wheels offset & backspacing.
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These rear wheels are:
Back Spacing: 7.5”(191mm) Offset: +45mm(1.8”),
puts the tire about 3/16" beyond the fender, which doesn't matter to me.
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ricky, my "daily driver" is a 2014 Mustang GT Premium. It is dropped only half an inch in the rear only. When competing at a NMRA event I mount a pair of Mickey Thompson ET Street SS #3491, 305/35-19, on a set of 11" rims with a +55mm offset. I have never had a clearance problem, except with my 1987 Foxbody, but that's another story/nightmare. With you being lowered, I am guessing the usual one and a half inches, you might have to install a set of "lower control arm relocation brackets." The red ones look really cool.