@britain - first of all, congrats on your purchase. Second, take a few minutes and RTFM.... Read the Fine Manual... It will tell you exactly how many gallons you have in your tank, and what to expect. It is well worth the investment of an hour or two. When my 2016 HRV hits '0', that means I have about a gallon and a half left, or probably close to 50 miles. I've driven to the gas station on '0' for ten miles, and still had over a gallon left over.
Next, experiment with the Econ on vs. off. I think most here will tell you it actually makes the mileage worse on the 2016-2022 generation HRVs. My suspicion is that the range should stay fairly accurate and shouldn't drop dramatically as you drive, unless you are going in stop and go traffic from having been on the turnpike, or start going up a hill. I tend to drive with my setting showing range as the default and it goes down gradually.