This is my first modification for GTA IV
Tutorial:

First: Backup your vehicles.img from location C:\Program Files\Rockstar Games\Grand Theft Auto IV\pc\models\cdimages

then open that vehicles.img file witn sparkIV tool and replace {ambulance.wtd, bus.wtd, firetruk, wtd, police2.wtd, polmav.wtd, polpatriot.wtd, predator.wtd, vehshare.wtd} with newone