Exactly what Loon said.
Whenever you download a car mod, it usually comes with both .wft and .wtd files, or just .wtd if it's a skin.
Unless it is a large carpack, these usually replace the vehicles.img file.
What is vehicle.img? Well, a very simple explanation is that vehicle.img works similarly to a .zip or .rar file. It is a file with many files inside.
What files are inside vehicle.img? The .wft and .wtd files I have been talking about all this time!
How do I replace them? There are many tools for that. OpenIV is a good one.
But remember, if you don't want to be banned from ArgonathRPG, you better stay away from changing .wft files (3D model, like I said before)
And again, .wtd files are fine and will
NEVER change the way a car behaves.
Also,
don't change lines in handling.dat, vehicles.ide, etc!
Trust me, I'm an engineer 