I owned a Lexus RX400h, which gave consistent results of 34mpg summer and 33mpg winter.

My current Lexus Rx450h (2009 with 70,000 miles) doesn't get much better than 32mpg.

It's puzzling as the 400h is rated at 34mpg and the 450h 44mpg. If I could achieve close to the rated mpg with the 400h, why cannot I achieve better with the 450h? My driving journeys and habits have not changed, if anything, I have become more diligent in trying to achieve the better economy.