When should you fill up your tank?
Wait until you are down to a quarter tank, but don't push it any further.
Doing this can extend your gas because you are hauling a lighter fuel load.
However, running a car with less than a quarter tank can shorten the life of the electric fuel pump. It also gives you the opportunity to buy more gas if you run across a bargain.
However, in cold weather, you run an increased risk of condensation in the fuel tank.