Did you get 55 mpg by taking the mileage and gallons actually used, or is that what the little TV on the dash told you? I have noticed most Prius drivers are going down the highway at 48 mph hunkered over the little tv screen, having little green orgasms as the screen shows larger and larger green bars.
Yes, I typically ride in the left lane in I95 in northern VA doing 50 mph delighting in how many conventional drivers I **** off. That's what gets me off, not the mpg I get.
But to answer your question on the MPG. I do use the screen for the most part as trying to get accurate readings using the ole fill the tank method and seeing how much is used between fill ups isn't accurate either on my car. The fuel tank has a bladder that expands and contracts with the amount of fuel in it. This was used for emissions. Unfortunately, it's elasticity (and mine) changes with the temperature outside the vehicle so unless I fill it up at exactly the same temperature those readings are suspect as well, at least that's what I've found.
In living in the northern VA area and driving here for many years I can't say that I've seen a Prius tooling along at 48mph. Most drivers here bought them to use in the HOV lanes. Why just yesterday on 3, I passed a newer Prius that was doing 60 in the right lane. I drive mine at speed or higher depending on how I feel and the flow of traffic. My car (or any that I've owned in the past) has never doddled along in any lane of traffic.
And that's it for the Prius. I know I started this conversation but let's veer it back on to the valve adjustment path (C14), please. Start a new thread in Open if you want to learn more from someone that's owned one since late 2004.