My RX400h is 15 months and 19000 miles. I used to drive one third on motorways,one third on roads and one third in cities.
Since the beginnning, my average consumption is 10.6 liter per km equivalent to 22 mpg. The computer indicates 10 % less consumption i.e. 10% more mpg.
The gap between both values could be due to the fact that the odometer is pessimistic of about 7 %, you can verify that thanks to the speed given by a gps (so when you divide the real gasoline consumption by the indicated mileage you obtain a value higher than the real one), while the inboard computer is, may be, calculating the consumption based on real gps mileage ...
As the speed limit are now in France equivalent to the british one, my piece of evidence is comparable to the other of this forum.
Thus it seems that the consumption performances given by Lexus are very optimistic and very far from the reality.
But of course, if you compare with the consumption of a X5 3.0i or other gasoline SUV, the RX400h is much better.
Regards
Jean-Pierre