> Half a tank is better. Less weight means better gas mileage.

Half a tank is the weight of a child (gas weighs ~0.7kg/L). The mileage improvement is pretty much non-existent. Your repeated trips to gas stations probably waste more gas than this retarded scheme saves, even ignoring the time wasted.

Full tank means fewer trips to the gas station. My time is way more valuable than the mpg savings of the half tank approach.

At the track, sure, but I try to never let my car get below half a tank in case of earthquake, personal emergency, power outage, etc.

More trips to the gas station means more fuel burned going to the gas station. Do you have evidence that the savings from carrying less weight outweighs this?