I'm actually just realizing how insane the fuel reduction is with more advanced engines. I was being lazy and not updating my exploration ships.
The route to fuel efficiency is through larger engines with reduced power. Higher tech engines are no more efficient than lower tech, they just have more power for their size. Fuel efficiency tech is nice but it's nowhere near as effective as making larger, less powerful engines.
The research for larger engine sizes and for lower engine power is pretty cheap.
The reason for this is due to the way fuel efficiency is calculated. It's based on the engine power squared, so numbers under 100% rapidly reduce fuel use.
Base fuel use is 1 liter per engine power hour. So a ship with an engine with 50 power, rated at 100% power, costs 50 liters of fuel per hour.
Take that same engine and give it 50% power, and it'll cost .25 liters per power hour. 50 units of power at that price comes out to 12.5 liters per hour.
It gets crazy as you go lower. A 30% power engine costs you .09 liters per power hour.
On top of that, larger engine sizes similarly get more and more efficient as they get larger. Those efficiencies stack. So you can easily have engines designed that take 3% of the fuel that a full power small engine would use for the same amount of power.
Fuel efficiency research is only giving you 10% better fuel efficiency at best. Of course, it stacks with the other reductions, so it's worthwhile, just realize in terms of reducing fuel use it's not going to have anywhere near the effect a lower powered engine would have.
There's a trade off of course. Large underpowered engines have a worse thrust to mass ratio, so even if you stack a bunch of them you're going to have a lower top speed. On top of that, large engines require more maintenance in military ships. It's still worth it to use them on your grav and geo scanners, you might need to add some msp storage to be able to maintain them though.
I tend to turn off fuel refining on planet first thing, and build some fuel harvesters to go sit on a gas giant. I have a tanker with huge fuel tanks take a visit to the harvesters twice a year and bring it back to earth, using order delays. I think you could probably set up the conditions on the tanker to fully automate this instead of using a delay, but whatever it works.