The electric assist is typically used while accelerating, especially up a hill. While cruising, there is very little load on the engine, thus, no need for "boost". You will find that on highway cruises, there is virtually no difference in mileage between an RX400h and RX330 or RX350. However, around town, the RX330/RX350's fuel mileage will plummet to 16 MPG or so. The RX400hs mileage around town is 28-30 MPG, almost double that of its non-hybrid siblings. We average about 25 MPG for combined city and highway. It is unlikely that any non-hybrid RX will achieve that kind of mileage under the same conditions.