So, I have a suggestion for improvement of the rank system. Currently, the system is very disproportionate once you get to Battalion Chief and it's next to impossible to go any further. I believe that the the final three ranks of Division Chief, Deputy Chief and Fire Chief should be adjusted to make it slightly easier to attain them.
I am recommending the following:
Division Chief: 100,000,000 Credits (100 Million)
Deputy Chief: 200,000,000 Credits (200 Million)
Fire Chief: 500,000,000 Credits (500 Million)
This would keep the structure of having to earn more credits in-between each rank than need for the last rank.