This is 78-79MPGe.
Slightly better than the Rivian, which is 78MPGe for DM Large (calculated to 136kWh pack using same method so the 131kWh above is pretty close).
The Rivian Dual motor number is not done with AT tires. (AT takes 352 miles down to 307 for Rivian R1T DM Large 20”, according to Rivian)
The Tesla number appears to be with AT, based at least on the presentation today (I can’t see what tires it is actually equipped with). The spare tire is claimed to be all terrain and there were multiple mentions of those tires.
Obviously CT tires may not “really” be AT. But it is possible “apples to apples” we are looking at 340 miles Cybertruck with AT, vs 307 miles for Rivian R1T similarly equipped. Seems a bit too good to be true, and subject to change with more info on what tires we are actually comparing. Especially since CT Cd is 0.335 compared to 0.3 for Rivian? Hard to understand - haven’t calculated frontal areas, hard to do exactly. CT is wider but shorter I think.
Or you could argue that with street tires you might get 380 miles out of Cybertruck (as an EPA rating - not actual miles of course).