This is a program to simplify the ability of modding the vehicle groups in GTAIV. First of all, it's "vehicle" group editor and not "car" group editor for the simple fact that there's a line in the file specified for boats. Now then, vehicle groups specify which vehicles spawn at which areas in-game.
Simply choose a vehicle or vehicles from the right list (make sure they're ticked), and click the button labeled "<<" which means add the ticked vehicles to the current group. If you wish to remove vehicles from a group, tick them in the left list then click the button labeled ">>" which means to remove ticked vehicles. As a safety measure, you can't add vehicles that are already in the list and also you can't add cars to the boat group and vice versa. Doing the latter makes the game crash during it's load sequence.
NEW in v1.2: Added function to remove selected vehicles from all groups in the file. To use, simply tick the vehicles to remove in the left list, then right click the list and select "Remove Selection from All", and it will automatically remove all selected vehicles from all groups.
You will need the Microsoft .NET Framework v3.5 installed in order for this program to run at all.