As far as I know, yes. In fact, pretty much all the hybrids have one.

There was a thread a few months ago in the FEH forum from an engineer, who was concerned when he kept getting lower calculations than the computer, so he did an experiment, and found it was right, and he was wrong, due to fill factors and the bladder.

I may be wrong about the HiHy, but that's my understanding as it is.

As for changing your tank data, yes, that's what the edit button is for next to your tank data, on the right, next to more. (After you've logged in if you're not)

First 4WD Hypermiler

Have you read the FEH FAQ?

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