Don't understand why rope would be part of the fuel. Typically you use a diesel to petrol (gasoline) mix of about 90% to 10% but 75%/25% would probably be better for balance. Could just add a diesel recipe to the chem station made from oil (presumably it is used motor oil as it's extracted from cars).
Could also add a bio-diesel recipe that consists of vegetable oil (from seeds, perhaps a use for tree seeds?), methanol (plant fibre, wood or corn), lye/sodium hydroxide (from wood ash* and murky water) and bottled water. Only wood ash would need to be added as a new ingredient - obtained from campfire in a separate output slot? and/or chem station and/or harvested from burnt wood in wasteland/POIs.
Finally, you'd mix 3 parts diesel or bio-diesel to 1 part gasoline for flamethrower fuel.