"When it's empty" seems like the obvious answer, but what if I told you that experts recommend keeping a minimum of ¼ tank of gas at all times?
Besides the obvious risk of not being able to refuel due to circumstances beyond your control (i.e proximity to a gas station), letting your gas tank get too low can have damaging effects on your vehicle.
Gasoline, though much cleaner now than in the past, can deposit sediment and debris at the bottom of your gas tank. So, the lower the gas level in your tank, the more likely that sediment and debris will enter and become lodged in your fuel pump or fuel filter. Both of these can lead to poor engine performance and reduced fuel economy.
Additionally, your car's fuel pump, which is located inside the gas tank, is designed to be partially submerged in gasoline. This allows the gasoline to act as a coolant and lubricant for the fuel pump. Thus, if your car's gas level consistently falls below ¼ tank, you are at greater risk of engine overheating or damage to the fuel hoses, both of which will require costly repairs.
Take the five minutes to pull over and refuel, you'll thank yourself later!