Had you cleared the Save folder contents after activating FotRSU, or used MultiSH4 to a new Save folder?
Generally speaking though, the FotRSU mod is more conservative in its fuel usage. The Gato might be capable of 12,000 nm travel without too much effort, and if you find the most efficient speed, might be able to get more from it. The game is very unforgiving if you run out of fuel, and while there are some inconvenient ways to get around running out, it was decided to go a little long, rather than a little short on fuel range for the mod.
