If you are looking at fuel cost savings you need to look beyond the cost per litre. You need to look at the cost per kilometre travelled.
Over several motor vehicles (2li v6, 2lit turbo, 1.8li IL4, 2li IL4, 1li v2 mc, 400cc single mc etc) in all cases I have found the cheapest petrol (cost per km) to be;
1. 95
2. 98
3. 91
4. 94E10
If you can still get 91E10 it sits at 5.
I have not done the costs for planes as it would be more work and I do not have one 🙂 but similar 'least cost per km' numbers would not surprise me.