handling.cfg is a file used by the Grand Theft Auto III era game engines for vehicle data. Being relativly simple to edit, many gamers tweak vehicle settings in order to change a vehicle's handling properties, such as accelaration, mass, centre of mass, ride height and braking force.
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 'accelaration' value and its 'mass' value. This led the heavy duty trucks and buses in GTA III era games to have higher top speeds than the sports cars. 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 (see below.)
- Ben 'Cerbera' Millard's in-depth studies of handling.cfg and modifications: http://projectcerbera.com/gta/3-vc/tutorials/faq