Anyone ran the R888R rears in a 315/30/20 on a lowered 10-14 GT500? Curious how they fit before I pull the trigger, and if they were any fitment issues. Wheel they will be going on is 11” wide +56 offset.
These are Michelin Sport Cup 2s in 325/30-20s on 20x11.5 wheels with +61 mm offset. Fitment is tight. I'd highly recommend removing the bump stop bracket as referenced here.
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I couldn't tell you if they run smaller or not. I think that claim is baseless and a generalization. Compare the specs. Just realize cross section widths are measured on a specific rim size. Differences in that measurement between tires can be attributable to wheel widths used in that measurement. Specs will indicate what rim width the measurement was made.
5mm less offset will put your tire closer to the fender. Whether the tire construction is such that it clears or rubs is anyone's guess. Your less likely to rub with a 30 aspect ratio than a 35, however. Removing the rear springs and jacking up the axle to the bump stops will give you an idea on clearance, just realize the bump stops will compress a bit further on a hard hit.
The bump stop bracket has no contact when the car is at ride height, but lift the car and let the axle drop down and the bump stop bracket will intrude against the sidewall and some even making contact with the wheel. Granted that axle motion may be difficult to achieve on the street, but it can happen.