A standard garden hose uses approximately 10 gallons/minute. Which means leaving on for 10-minute = 100 gallons of water. 

Ways you can save water while washing your vehicle is by using water-less products, use a bucket and a hose with an automatic shut-off nozzle when you wash the car, or take your car to a car-wash that recycles.

Other ways to conserve water:

• Use a broom, not a hose, to clean your driveway, deck or patio.
• Cover pools and hot-tubs to reduce evaporation.
• Save the water you use to wash produce or cooked in to water plants.
• Reuse the water collected within your dehumidifier to water plants.
• Instead of rinsing your razor under running water, use a bowl of water to rinse your razor.
• Collect the water from your shower or tub while you are waiting for it to warm up and use it for your animals, toilets or plants.
• Install a sink-to-toilet water system or grey water system
• Turn off the sink when brushing your teeth
• Fill a plastic water bottle and store it in your toilet tank to use less water with each flush.
• Capture and collect potted plants water that goes out through the drainage holes for later use on the same plants or garden.