Please don't let the hose run. Instead, wet the car thoroughly, then turn off the hose while you swab the car with soapy water from a bucket. Use the hose again for a final rinse. A trigger nozzle is best because it turns off automatically. If you have a swimming pool, get a cover for it.
Evaporation can make hundreds, even thousands, of gallons of water disappear.
An average-size pool with average sun and wind exposure loses approximately 1,000 gallons of water per month, enough to keep a family of four in drinking water for nearly a year and a half. Remember, you can also recycle the water for use on your lawn or garden.