My sister and I both use the same car. When we started to share, we were both students and never had more than £10 to pay for petrol. Now that neither of us are students, we still only put a tenner’s worth of petrol in because we figure, “What is the point of one filling the car up for the other to get the benefit of driving it?” – commonly known as “sisterly love”. Of course, this means that we are endlessly having to stop to put in petrol.

Is there an optimal amount to put in the car in our situation, or, perhaps, a better way to deal with the predicament, bar getting a car each?
Lucy