1967: First year FoMoCo offered an auxillary fuel tank for F100/350's, used in conjunction with in-cab fuel tank.

Manually operated fuel tank selector valve on floor adjacent to the drivers side of the seat switched the tanks. Toggle switch below dash switched the dash fuel gauge from one tank to the other.

For many years, aftermarket companies offered single and saddle auxillary fuel tanks.

Thru 1955: 6V positive ground / 1956 thru today: 12V negative ground.