Unbreakable cars are vehicles that cannot sustain any damage. This effect is achieved by applying a minus value for the car's health when compiling the vehicle for use in Midtown Madness 2. There are many vehicle mods which use this feature, including some which offer both breakable and unbreakable versions of a car.
In July 2012, UNLEASHED released the IDC Mod (Incredible Default Cars) on the Midtown Madness Archive, which modified all default vehicles to be both unbreakable and ludicrously fast. However, this mod is still in beta and the download link currently appears to be broken. Also, the mod appears to have been created using the MM2 Ultimate Tweaker, so it does not contain any original content.