I just purchased an 87 F-150 with dual tanks. I filled up both the day I got it and I put around 15.5 gallons in the rear tank and around 17 in the front tank. Although I was not totally empty in the rear tank but I was in the front tank. I also noticed a sizable dent in the front tank when looking at the underside of the truck so the capacity may be more that I put in them.