I have a 1973 Ford F350 super camper special. It has the 390 engine. The truck has dual fuel tanks. One is under the cab inside the frame and then the other is at the back where the spare tire goes on the regular models (this truck has the spare inside on the side of the box).

My question is; which tank is main and which tank is aux?

Thank you.

 

The rear tank should be the main tank. I don't know that it's a hard and fast rule, but I've found that if the pick-up came with a single tank under the bed, the rear one is the main. If it came with a single tank in the cab (Like a Crew Cab or High-boy) then the main tank is the one in the cab.

 

Looking at my truck the main tank is the one in the rear.