You need a commercial grade inexpensive pressure washer that can generate a nominal 1200 PSI and 1 GPM at least. Cars do not require heavy duty pressure washers. In fact, too much pressure can damage the paint and leave water stains.
The Precautions Before Pressure Washing
Before you begin, first park your car away from the house or garage. You do not want to damage the walls of the garage or the outside of the house with the pressure washer.
Also, if you have scratches on your car, you should cover these areas with tape so as to prevent water from percolating on the exposed metal part and rusting it from the inside.