I'm using LFS Tweaker from https://www.lfs-tweaker.com/ and it's not working correctly, I try to increase the power but then the automatic shifter stops working, and the AI can't shift past first or second gear. Why is this?
Because the rev range is different and LFS' automatic shifter is waiting for the right point to change gears, which it hasn't got to yet. Increasing the rev limiter should fix this issue.