We’ve used a number of different cars via ZipCar over the years, and have always paid attention to gas mileage on long trips (we’re computer nerds - we just like thus stuff.) We found the readouts to be quite accurate. Our current Hyundai Elantra has also proven to be accurate. No reason they shouldn’t be, with modern fuel injection systems controlled by computers metering the fuel delivery constantly.
All through my childhood my parents had an annual logbook in every car we owned and kept track of every gallon used and mileage. You could get the little books from gas stations. At the end of the year my dad always ran totals. My mom no doubt still has all of those records somewhere.
We also kept special logbooks for vacations, with notes on campgrounds we stayed in, when we got up, miles traveled, any sightseeing that day, temperature in the morning, stops for groceries, where and when we went to Sunday Mass, and a lot more. My dad was so geeky.