handling.cfg (or handling.dat or handling.meta) is a file used by the Grand Theft Auto III era game engines for vehicle data. In Grand Theft Auto IV, the file is known as handling.dat and calculates more factors. In Grand Theft Auto V, the file is known as handling.meta.
Being relatively simple to edit, many gamers tweak vehicle settings in order to change a vehicle's handling properties, such as acceleration, mass, center of mass, ride height, and braking force. The file is often edited when installing vehicle mods.
It is a generally held opinion that vehicles tend to have an unrealistic amount of traction, and hence have an exaggerated capacity to take corners at speed. This also renders sustained drifting almost impossible. The game engine calculates a vehicle's maximum speed based on its acceleration value and its mass value. This led the heavy duty trucks and buses in the GTA III era games (except Grand Theft Auto: San Andreas) to have higher top speeds than the sports/supercars. The default brake force values on almost all vehicles are so high that any use of the brakes causes all four wheels to lock up, making it difficult to slow for corners. Those who seek a more realistic driving experience may tweak these settings.
Ironically, GTA IV's vehicle's handling system has been argued by some to be too realistic due to heavily decreased traction and more subtle oversteer.
Handling.cfg files on the Android versions of the games are modifiable, provided the APK has been edited also (in most cases). iOS allows modifications from the get-go; jailbreaking is required, though.
These subpages contain the original handling.cfg files from their respective GTA games: