One of my modded cars made my game not work. So i reset it all i downloaded a vehicles.img and i replaced it. Theres now no cars on the street? Can someone direct me to a download that will work?

Do you have GTA IV on Steam? With steam, you can right click on the game in the library, go to properties, hit local content and press verify integrity of game cache, this will search for altered or corrupted files and restore them.

You will, however, lose custom handling files, visual mods, etc, but you can reinstall those.