hello, I have long time no play LFS, I just back & found that the steering speed of mouse is very fast now, it turn dead to dead even I just move little bit, how could I tune it slower?

It's not possible to adjust the mouse sensitivity in-game.

Here is a related improvement suggestion thread: https://www.lfs.net/forum/thread/95111-Steering-sensitivity

The car wheel angle per mouse movement is still the same - only the steering wheel rotation has been increased in some road cars.