You are 100% right Hyronymus!
Everything will be editable through scripts (and the like, etc), so to add a new fuel type you would just have to code it yourself.
Default settings should be those which are seen to be complete but not exhaustive.
This doesn't answer some questions, such as...
Do we want fluctuating fuel prices? (If so, what are the economic models, or factors which we will have to consider in deciding the price.)
How are we going to relate fuel usage to each engine. (Will we use just a simple power = petrol, or something more complicated which depends on some engine parameter, eg efficiency)
Dave Worley wrote:I know it's almost petty, but this difference is 6 pence a litre in Britain (diesel being more expensive - somehow).
(Off-topic) A diesel engine will (in general...) consume less fuel per mile than a petrol engine. (Which is why you see those shiny new Diesel BMW's). So perhaps the difference is zero, or maybe diesel is actually cheaper?
From Wikipedia: "The density of diesel is about 850 grams per liter whereas gasoline (British English: petrol) has a density of about 720 g/l, about 15% less. When burnt, diesel typically releases about 40.9 megajoules (MJ) per liter, whereas gasoline releases 34.8 MJ/L, also about 15% less."