All of your settings are correct?
In game; turn off vsync, set fps to your monitors max refresh rate (in your case, 75fps).
In AMD graphic settings; set fps to your monitors max refresh rate (make sure your refresh rate/fps is the same both in game and outside the game), turn off vsync, turn on freesync, turn on adaptive vsync (adaptive vsync is available in my nvidia graphic settings so i use it. I'm not sure if is available in amd graphic settings. But if it is then I'm not sure if you need it. If you have freesync monitor and compatible gpu then you do not need to enable vsync or cap your fps to a fixed setting. Freesync/gsync keeps your gameplay smooth).
Monitor settings; turn on freesync, use displayport cable (if you're getting 75fps then i guess you are using one already), turn on game mode.
I'm not an expert, just trying to help.
I have a nvidia gtx 760 and a freesync monitor @75hz. In Automobilista get a stable 75fps with an ocasional small studder with mostly maxxed out graphics. But I still can imagine gameplay will be even another once I upgrade and convert to an AMD gpu.