The situation with fuel trading is weird. How is it weird? Well, let's look at it in more detail.
Your space ship can hold between 80 and 120 tons of fuel in the fuel tank. This kind of fuel can be bought for around 1 cr / ton and sold for around 0.5 cr / ton.
However, there is also the sort of fuel that can be put in a cargo hold. One ton of this 'fuel' takes up an entire cargo slot and can be bought via trading for 10 cr / ton give or take a bunch.
It just doesn't make sense that these fuel categories are so different, but are apparently supposed to be the same thing. It is also strange that one ton of (fuel tank) fuel can take a tiny scout vehicle exactly the same distance as a massive battleship.
I do have some ideas for sorting this tangle out, but the fuel situation is a bit complicated so some compromises and decisions may have to be made.
1. Change fuel tank display for ships to a percentage based one. Ships will have from 0 to 100% full fuel tank. One move with pure fuel will usually* use up 1% of the fuel tank. * Ships that have small fuel tanks will use up a bit more than 1% per move. Ships that have, relatively, large fuel tanks will use up a bit less than 1% per move.
2. Fuel tank capacity (kg) will vary for each ship hull. A battleship might have a 10 ton fuel tank, a scout ship might have a 200kg fuel tank. Price to refuel will vary accordingly.
3. Trading prices and 'gas station' prices for pure, refined, fuel should match.
4. You could purchase a 'fuel storage kit' for your ship. This allows you to store fuel in your cargo bay. Without it you can't trade in fuel to / from your cargo bay. You also can't store raw fuel from gas giants in your cargo bay without it.
5. If you have a 'fuel storage kit' on your ship you could also top up your fuel tank from the fuel in the cargo bay. This would include fuel purchased by trading.
6. 'Raw fuel' wouldn't be a separate trading commodity. Instead the value of impure fuel would be reduced according to how impure it is (plus a penalty for the processing required?).
7. Note that item 5 and 6 leaves you potentially trading in fractional units of 'fuel' commodities. Which might be tricky to implement. It might be easier to eliminate fuel from commodity trading and instead only trade in it via the 'refuel' station and the 'administration' location.