Rivian Efficiency

What can I expect to get KWH/mile driving a R1T down the interstate at 75 MPH? What about in the winter?

I recently drove a R1T and really liked it. The sales person told me Rivians get the range that is advertised and the cold don’t affect them much. I’d like some real world stats before I believe this. I have a Lightning and am a little unsatisfied with the efficiency.