I own a RX400h and a Toyota Auris Hybrid Synergy Drive, and I am Chartered Industrial Engineer, so understand that I am qualified to comment on what follows.
The average fuel consumption listed on the catalogs are just that, average, so you have to take into account the type of use we make of the vehicle and the terrain by which we drive -and of the latter have not seen anything written -.
For example, the best performance compared to conventional cars is smooth terrain, in driving periods above 30 minutes, or shorter, but consecutive -and traffic jams, or with many stops, p. eg. due to traffic lights. And in temperate climates. And at moderate speeds, less than 50 mph.
That is, the best fuel consumption is most probably achieved by taxi drivers in big cities.
The worst can be found just in the opposite situations: Many climbs and descends, cold weather - even worse than the hot, including the overconsumption by A/C-, driving for less than 30 minutes separated by hours, because the ICE runs until the oil is hot, and that hurts the MPG average. And at high speeds, above 50 mph.
And above all, avoid palces with steep orography, as while climbing it consumes as a conventional car and in the descends the bad thing is not that you cannot recover the braking energy, a sit "overflows" the small battery, but that you do not have a way to slow down with the ICE, unlike even in a automatic, where you can shift to "2" or "1".
And no, the "B" is not enough. The solution would be to have a "B2" and "B1", to avoid eating up brakepads quite quickly!
So in small and steep islands, you will notice that hybrids are not purchased by the locals but instead rented by tourists willing to pay a premium or that might have been brought in from mailand.
It is a great technology, but if you do not use in its optimal conditions, you will be driving a technologically overloaded (and overpriced) car without obtaining back any economical nor environmental benefits.
