Why can't I change my final drive?

I cannot seem to change the final drive ratio on my WTCR car. What am I missing?

 

You aren't missing anything. They are fixed in the series.

 

Thanks Paul. On which cars or under what circumstance can you change the gearing on a car?

 

The cars that belong to an official series are normally locked as per the rules of those series. I am not really sure which ones you can unlock to be honest.
If it is in MP and the server has fixed setups then you won't be able to change anything.