Thanks for the report!
In this video,
they towed relatively tall trailer, getting about about 0.9miles per kwH with both F-150 Lightning and the Rivian Pickup during a longer but roughly comparable trip. (Their Rivian had the 130kwh battery pack, your SUV might be equipped differently.)
Rivian was showing around 1.6 miles per kwH and very high remaining range while towing in the upper half of the battery, but then dropped performance numbers hard (to around 0.6 miles per kwH, as the battery become more run-down. F-150 did a better job of projecting remaining capacity over the trip as a whole.
Their round trip included larger altitude gain/loss than yours, and the tall trailer box was much worse (for wind) than towing a low-loaded flatbed or TM. But you should take care on the way back: If your Rivian showed 60% remaining on the way out, you probably WON'T MAKE IT HOME without charging.
Based on their testing, I'd SWAG the real-world performance to be around 1.2 miles per Kwh. The Ford did a better job for Edmunds because it asked for specific trailer weight and size information, while their Rivian pickup only asked "are you towing"?
Edmunds/YouTube testing saw bad "remaing range" with a Rivian pickup.
Hybrid Mode
