From the BeamNG documentation: Weird shading on imported models Some models might appear with shading issues when imported into blender, most visible around the doors. This is because those models were exported using 3DSMax, which uses a different method to define which edges should be made sharp. To fix this issue, you can apply an autosmooth of around 45-60 degrees. While this won’t give you the exact same results as the original model, it should be close enough for modding purposes.
Also, depending on exactly what you're making as a mod, do not export the entire car if all you're making is a handful of parts. This has the effect of overwriting vanilla content, which can cause weird issues, especially if the devs update the car.
