With the new battery options, it'd be helpful to know what battery and ending SOC for each section of the I-80. I'm really hoping a Large Pack R1S can get to Truckee without stopping in Sac. However, cold weather + roof box + the ascent might make that tricky...
So it sounds like you were able to get the box onto the rivian cross bars. Sounds like good news as I have a pretty lightly used Thule Atlantis 1600 XT that I would like to continue using.
I worry about the local roads on hills than the main throughputs when going to Tahoe.
Heading up to Donner HOA area is a pain even with M+S and it's snowing. Saw a Tesla X slide after coming to a stop when going down back to town. EVs are bottom heavy and the tires can't hold as well on a slope.
Not sure how reliable this site is but it's listing the OEM 20" size as 55 lbs and 126/123R
https://tiresize.com/tires/Michelin/LTX-MS2.htm
Don't see 21" or 22" in the list