Lexus RX400h Amazing Fuel Efficiency @ 100km/h

cruising @ 100 km/h with my RX400h in a levelled long highway tunnel, cruise on, AC off, unleaded 95 gas (standard gas here). Car runs constantly between 5.2 and 5.7l/100km (41 and 45 mpg), later dropped as low as 4l/100km (58 mpg), really amazing. Overall MPG of the car currently is 8.0l/29mpg at the moment. I don’t do hypermiling.