I have a 12v water pump. 15gallon capacity tank. It shoots out 1gallon per minute(gpm). I use it instead of the pressure washer due to cost of big tank and pressure washer. Way i wash cars i take my two five galln buckets one with soap another with water to clean mitt. Wetting the car and washing away the soap i use about 4 gallons tops per car and no soap whatsoever is on the car. My question is those who use a pressure washer is a car easier to use with pressure washer? How much water do you guys use per car?