I saw you added config for fuel in numbers (MJ/items/stacks).

But could you add an another option: slots percent (slots%) - target fuel slots:
if car has 2 fuel slots, and this value is 0.5 - the car gets 1 fuel stack
if car has 4 fuel slots, and this value is 0.5 - the car gets 2 fuel stacks.
if car has 2 fuel slots, and this value is 1.5 - the car gets 3 fuel stacks: two in fuel slots, 1 in inventory (if it exists, and has empty slot).

Note: Different cars need more fuel than others. By specifying 50% (of slots) I am specifying 50% for the standard needs of each vehicle, not the minimum fuel level.

Could be possible, but I think slot% is too confusing without your lengthy explanation. I could also do something like Burn time (like 5min = enough fuel to drive around for 5min uninterrupted)