This tweak does not change vehicle behavior (speed, acceleration, shock absorbers) in any way.
It only removes boat-like wobbling when making turns or braking.
It wasn't meant to be realistic or anything, I just find the vanilla car suspension too soft for my taste.

Installation:

Locate your GTA IV installation directory.

Copy handling.dat file found in the zip and replace it with the one found in Grand Theft Auto IV\GTAIV\common\data.