I can get more gas in the tank when the front is lower since the filler is towards the back of tank. Therefore, the filler doesn't get fuel until nearly all the gas tank is full. When the front is higher, the fuel may reach the filler before the front part of the tank is full.

There's one station that has a definite decline in the concrete pad. In fact, with the Sub, the front tires actually drop off the concrete onto the asphalt, which is slightly lower still. I bet the front axle is about six inches lower than the back. It was here that I put in $85 worth of gas in that thing.

Hope this makes sense.

 

53.3 on FCD, 42.8 calculated

What probably happened is that my last fillup 'clicked' early. However, the last few times, actually, the tanks has been underfilled I've noticed... but my mileage has been fairly close to the meter, usually within 2mpg either way. It's possible that this one actually slightly 'overfilled' the tank. Either that or it's calibration was just way off.

 

Variations in when full is full are really noticeable in the Prius II in the winter, because of the tank bladder that has different P/V curves depending on the ambient temperature, and is markedly affected by pump fuel rate.

A couple of times, I had to manually squeeze the handle the entire fill-up to avoid automatic click-offs. Not a big deal, and as others have said, the lifetime MPG tells the truth a tank or two later.

I have not bothered to prove it to myself, but I am pretty sure that the car's MPG calc is quite precise, if not accurate. If you are interested, just measure the delta of calculated from car based on five tanks or so, and use that on each tank. People have reported for the Prius II that MFD is ~ 1.5% over real.