1. The subwoofer it's self, and the amount of air it moves (cone area x average xmax) 2. Your amp, amount of wattage running to your sub. 3. Your enclosure plays a huge role, it's impossible to say what your hitting without knowing what kind of box. 4. Your vehicle, cabin gain also plays a big part in it, and the same setup may sound different in a different vehicle. 5. Your vehicle's charging system can actually play a part in it as well, determining the voltage going to your amp, which in turn determines the wattage produced (if you have an amp with an unregulated power supply)
To sum it up, it's simply impossible to determine how many db's a person will hit, even if all the factors are known. There's just too much stuff that can effect the outcome, and nobody can honestly give you a close number. The ONLY way to determine what your hitting, is to actaully get your system metered.