I had my buddy in the wheel industry look into 18", but that failed. 🤦♂️Yellow line is a trace of our caliper on a 2D of a 18" wheel barrel.The Saint is correct the OEM offset is +48. My issue with the OEM rim for rock climbing is sliding off a boulder and damaging the wheel because the spokes have no offset. This is shown in a you tube video.
My hours of research have resulted in not too many options to test a wheel tire combination forwheel well clearance. A local 4 wheel specialist will order one wheel to try but will not mount a tire.
Tire rack will allow me to return wheel and their mounted tire if not driven on, but their minimum offset is +20. I would try the fit at their pickup center in Denver and not drive on it for an easy return.
Rivian Service Tech says trying to raise the vehicle to off road just for clearance limits the speed to 50 mph, then the Rivian computer will take over and lower the vehicle, whoops. Rivian ST also said that the OEM Pirellis are molded with a camber and that an aftermarket tire will not track nicely on the highway. In other words, the Pirellis are custom made for the Rivian weight and driving modes. This is first hand knowledge from Rivian technicians, not social media.
I sure hope someone actually posts first hand information on their success getting and using a true off road rock climbing wheel and tire combination.