#23 in this picture is the fuel reserve valve. The Fuel reserve was in models from 1952 to 1961 in the VW Bus, the VW Beetle, and the VW Karmann Ghia. When the lever is in the UP position, the tank will only drain down to the top of the reserve level (this will leave a little over a gallon of gas in reserve). If the lever is in this position and the car starts to sputter (running out of gas), you can move the lever over to RIGHT position with your foot to allow the “Reserve” amount left in the tank to run to the engine. The most important thing to remember is that you MUST return the lever to the UP position after your next fill-up. Otherwise, later – if the engine starts to sputter and you move your foot over to move the lever to the right, you will discover that you have already used up your “Reserve.”
It was used when there was no gas gauge in the car. When the engine started to sputter you would turn a little knob under the dash and, viola, you had a few more liters to get you to a gas station.