I just traded my Prius in for an RX400h. Although I've only driven it a couple of days, I'm getting about 28 mpg but haven't driven it on the highway yet. Here's what I do: Say the speed limit is 45. Gradually get the speed up to 45, then coast a minute, then lightly accelerate keeping the speed about 42/43 miles per hour. You will notice that your hybrid motor is running and you are getting good mileage. Prius-owners call it "stealthing."