The fuel tanks on the designs may seem large, but in comparison to the gargantuan harvester components, it still comes down to a tiny percentage of the designs, so it is not bad to have a year ration or so on board.
Normally I just use the default fuel deposit order and have the nearest moon at the gas giant made into a colony for that purpose only. This means that the trips are really short, and it avoids having any harvester fly to random colonies from system spin or civil mining site opening etc., which annoyed me in the past. From that colony I can abduct fuel after my own calendar - whenever it is needed or I think of it.
However, I also had the situation where my first found good gas giant didn't
have any moons, and neither was there a close enough body in the system that could have served as substitute. In such
case, it is good that you don't have to change the design for more fuel storage, because you can still let it work for a year, then send a tanker with saved order template once the log starts complaining.
Edit: in reference to the discussion of fuel consumption , the crippling fuel shortages I've been running has made me consider efficiency a little more than I usually would.
Ignoring power multiplier for a moment, if my commercial fleet is all 30% power level then Fuel consumption for a ship is based entirely on power level, no matter how many engines I pile into a ship the increased speed just means higher consumption, or vice versa. So in theory when the ship arrives at the destination Fuel use is the same right?
But taking into consideration the actual payload capacity of the ship can give greater efficiency.
Like here:
When engine and design efficiency is eliminated as a factor (because engines are of same power+size+tech, and the designs use same engine percentage), then fuel cost is determined by how much tonnage of ships in total you have to move around, not the size of any individual unit.
You can analyze it as this:
1. The more engine percentage you use, the less is there for actual "mission" tonnage.(can also be shipping I guess)
2. You thus need a larger ship to ferry around the same amount of cargo holds(/or whatever equipment).
3. A larger ship means more tonnage in total to propel around, and thus steeper fuel cost despite same amount of holds.
E.g.: A 20 holds freighter with 40% engine dedication is pretty much exactly 1 megaton large (400kt being engines). With 50% engines, it grows to 1.2mt (600kt engines), so you pay 20% more fuel for the same accomplishment of 20 holds material shipment.
..But wait, it is also 25% faster, so given you have an open ended order and the freighter constantly flying, you actually end up paying 1.25*1.2= 50% more fuel on the same time scale, and that for just 25% more speed/availability.
Large engine dedication is hugely costly. You really have to think if it is worth it.
If you are interested, I once researched the rule that lays behind this, which is this:
e2 an e1 being the engine dedication factors, like 0.4 for 40%. t2 and t1 are then tonnages of course.
Calculation example: You want to upgrade your 6kt and 33% engine percentage destroyer towards using whooping 50% as it is your only way to match a new enemies' speed. This fills into the equation as (1-
1/3 / 1-
1/2)= 4/3, so 33% more mass (8kt) if you want to keep the equipment constant.
You can then get fuel efficiency difference by calculating t2/t1 * e2/e1, which in this case would mean exactly double the fuel cost despite only bringing the same amount of weapons into the fight.(a little more armor though)