The car suddenly jumps while driving on the road.

[GodToRun]

The car suddenly jumps while driving on the road. The same thing happens if you combine the mesh. How do I fix it?

[JustInvoke]

There are a lot of things that could contribute to this including the car rigidbody scraping on the road mesh and hitting a bump or seam between geometry (most likely), over-tuned suspension settings, over-tuned wheel settings, etc. It's hard to figure it out without a video or demo scene to try.

Combining meshes wouldn't necessarily help unless the interior geometry between the combined meshes was removed and adjacent vertices welded together which is fairly complex to do. I've been working on a tool to do this for a different project and maybe I can share it in a different repository once that's ready.