I don't know if the title says what I want to say. So, i'm trying to make an automatic gear changing system for different types of driving. For that I'm using c#, but I don't know which library to use. I mean I don't even know if it's possible to make that. So that's why I starded a new thread. Can I do it with the InSim library?

You can't actually control a car via InSim, OutSim or OutGauge. The only way of controlling a car through some external application is to generate keypress events for keys that are bound to some actions in LFS - simply put, your application would have to "press" the keys as if it was the player. There's at least one example how to do this in C++/WinAPI.

Quote from amaru_amaru :

So, i'm trying to make an automatic gear changing system for different types of driving.

You might be interested in this.

I tried the SendKeys.Send function but it didn't work ...

SendKeys are not gonna work for you, it's suggested to use SendInput instead. If you're writing in C#, you'll probably be interested in this