The problem is that a jerrycan can hold a lot of fuel (20 litres). A car spends one doze fuel per zone sector travelled. That's one dose per mile (I think a sector is one squaremile).
How much is a doze? Well, I don't know, but if we assume that a car (old car - 80s model*) can drive around 7 miles per litre. If we add in some bonus driving, we have 5 miles per litre. This mean that a jerrycan have 100 doses.
This is what I assume. I think 100 doses is a nice even number. I sometimes add a dose extra if I think they have driven around a sector more than usual. I also rule that a if a player converts the engine so it runs on alcohol, it needs upp to 3-4 doses of alcohol (or even less) instead of just one (depending on skill roll when converting).
* My games have a "cultural stop" at the 80s (the times when the world in my game started to go to hell), so artefacts and scrap and such is usually in the form of 80s things.
“I’d say thistles, but nobody listens to me, anyway.” - Eeyore