My fuel production is decreasing. The production rate is 78.0/h and the troop consumption is 98.0/h..

If you want to be able to to keep producing a lot of fuel then you can either increase production or reduce the troop numbers (vehicles). But this is not necessary if you are able to keep earning rewards.

Increase production with upgrades to the buildings, or look for management heroes to station. Reduce troops by letting them get wounded and then dismiss them from the hospitals.

However the game does not let you run out of resources and will have a minimum value of fuel below which the consumption will stop. You can leave your fuel at this point and earn stored fuel item boxes in rewards that are kept in your inventory (not the warehouse amounts at the top).

Keep these fuel boxes until you nee them for upgrades / research etc and use them as needed. If you are playing regularly and managing to complete daily challenges and other rewards you will be able to stock up on these stores of fuel to use as needed.