Abstract: An auxiliary fuel system for installation beneath the rear portion of the floor pan of a passenger car. The auxiliary fuel tank in the system is resistant to leakage even when the car in which it is installed is subjected to a rear end collision. The system increases the survivability of the auxiliary fuel tank by the use of transversely extending support straps which permit the longitudinal forward and downward movement of the auxiliary tank during said collision, to thereby help prevent its being crushed. The tank's ability to move downwardly as it moves forwardly during said collision is enhanced by the tank being provided with a sloping front wall which deflects the front of the tank downwardly as it encounters a transverse member in the car, and by the tank also being provided with a large, relatively shallow fuel storage area designed to bulge during said collision, thereby also tending to force the tank downwardly away from the car.