Well I can't estimate your tank capacity, but for most people, this just isn't an issue. The Frances is a trail from village to village. On average, they are about 8-10 Km apart or so, and virtually all of them have a bar where you will stop in for second breakfast, coffee, water, lunch, ice cream, a sock change, or just to use the facilities. My wife was VERY concerned about this before we left. It turned out fine, except for the one 15+ Km stretch on the meseta where she needed to take steps......off the trail that is.
Buen Camino