This allows you to accelerate your car by pressing a key as fast as you want. A specific speed will be added to your current speed and if you wish you can set, that the speed, which will be added will increases all the time. May be it sounds a bit complicated, but it isn't.
You can control speed and keys via nitro.ini. Default: [SHIFT] accelerates your car. The key over TAB displayes the current speed that will be added and F12 reset the speed.
You need C++ Scripthook to use this mod.
Just copy the nitro.asi and nitro.ini to your GTA IV folder.