The drag goes up with a cubed factor, so, yes throw in the speed and the grades, that range is possible. The car was designed for city operation, with a possibility to extend that with the REx if needed. It was not optimized for longer trips at high speeds.
Take an ICE at say 55mph versus 80mph, and its efficiency will drop considerably, too. An ICE doesn't need to burn extra fuel to warm the car, either...the REx does. FWIW, the BEV is more efficient with heating because it has a heat pump, but on the REx, every watt of heat you need, decreases your range one watt.
If you want to preserve range, slow down, and if available on yours, more seat heater versus cabin heating can help.
2011 535i x-drive GT, 2014 i3 BEV