Earlier quoted context omitted.
Unless your petrol station is on one of your regular routes, it's false economy not to fill up completely when you have an empty tank, otherwise you are incurring an extra trip to the petrol station. Of course if you can't afford a full tank, and are living hand to mouth each month/week, then you are in the poverty trap where everything is a percentage more expensive for you overall.
But the efficiency is lower driving around with a full tank.
Found this[1] from a quick search.
[1] https://www.carsforsale.com/how-to/5-simple-reasons-to-keep-....