Even though we fill up the car perhaps once a month, we need to do more planning than you would think at first. For example, this past weekend we were thinking that the car was pretty low on gas and we should fill it up, but we're planning on going to Spokane next weekend. We decided it was low enough that we should fill it, and it'd still be close to full by the time we leave. But that's not where it ends.
The other consideration is that fill-up time is when we take the opportunity to run the car through the wash. However, we had just taken the car to the shop and they washed the car for us. The executive decision was to fill up, don't wash, but wash when we fill up after getting back from Spokane.
Yes, pretty simple reasoning, but it makes us feel like we were able to make decisive choices. We take them when we can get them.
Trying to figure out how low to let the car get (remembering that we don't want it too low, especially since we don't drive that often) reminded me of when my parents got a Peugeot diesel, must have been when I was in high school. We picked up the car, drove it around for a while, and then went on a trip. Seemed like we had driven quite a while, but the fuel indicator still read "Full". My dad was so happy because we must getting such good mileage. We had already gone a couple hundred miles, and doing the liberal estimation, we must have been getting well over 100 MPG at a minimum.
We kept driving, but even my dad was thinking that the gauge should have been lower by that point. We found a station which sold diesel, filled up, and found the tank was about half empty. Needless to say, we figured out when to fill up by mileage for the rest of the trip, and had the gauge fixed after we returned.