In the bed?

My 77 has holes inside the right rear of the bed for a bed mounted spare, but the brackets and spare have obviously been removed prior to my ownership. There is no spare mounted under the bed and I don't currently carry a spare in it.

As far as contributing to the confusion of this thread, my 75 F150 has dual tanks and if feeds from the rear when the main tank is selected. When you switch it to aux you can hear the solenoid under the cab click on and it feeds from the forward mounted tank. If you leave it set to aux and cycle the key from off to run back and forth you can hear it click on and off. Neat huh? Not really, but that's what it does.

aux sucks from the rear tank, main sucks from the side tank. The rear is smaller than the side tank. this p-r-o-b-a-b-l-y makes the main the larger and the rear the aux since it is smaller. There is a small section of rubber fuel line that probably needs replaced on your trucks that connects the selector valve to the feed line from the tank. you can switch them anyway you want.

They both pull from the rear tank???

NO. Read it. the rear is smaller. the side is larger. main=larger aux =smaller. R-E-A-R= smaller. Been filling my 1978s for 17 years, as long asa I have had a driver's license. There is a electric selector with 3 rubber hoses. one to front tank. one to rear tank. one to engine. Where on earth did you get they pull from rear tank.

Call me silly but I got it from post #47. You wrote it.

https://cimg7.ibsrv.net/gimg/www.for...1e8b9cd813.png
1976 F100 thru F350, P & M Series Owners Manual
It could be Ford switched Main/Aux designations in 1977. Rear might have become the auxiliary then.
1976 Owners manual shows;

Your 100 percent righton my 78 f150 the front is the main the rear is the auxiliary on my 1977 f250 the front is the auxiliary in the rear is the main

way to bring a 15 year old thread back.