In the past, I've opted for using a garden hose to wash our car, a small sedan. I use a shut-off nozzle, so the hose is not running continuously. However, I'd say that the hose would run for about 5 to 6 minutes for one wash. I'd like to add that according to several estimates, my water use for using a hose is below the national average by as much as 5 minutes of run-time.
The average garden hose uses about 10 gallons of water per minute. For my use, that means that I was using about 50 to 60 gallons of water each time I washed my car. The other day, I chose to use a bucket, 4 rags, liquid dish soap, and water. My bucket holds 3 gallons. I filled it 2/3 full 3 times, for a total of 6 gallons of water and did not use my hose at all. I used 1/10th of the total water to wash my car compared to how I would use a garden hose.
My 3-bucket method worked very well. The first fill included a small squirt of liquid dish soap. You really only need a tiny amount of soap, otherwise you end up using more water to rinse the soapy bubbles off of the car's surface. I used rag #1 to dip into the soapy water and wipe/scrub onto all of the painted and glass surfaces of my car (I didn't wash the wheels or any of the underside of the car). By the time I had made 1 circuit, my bucket of water was empty. I set rag #1 aside. For the second bucket, I filled it with 2 gallons of clear water and used rag #2 to wipe/scrub away dirt and some of the soap. When my bucket was empty, I had made a little more than 1 circuit around the car. I set rag #2 with rag #1. I then filled my bucket with 2 more gallons of clear water and rinsed the car once more, using rag #3. The car was clean enough by the end of this water. I used rag #4 to dry the entire car, beginning with the windows first, as they will spot if left to air dry or streak if the rag isn't dry enough. The whole process took about 20 to 25 minutes and left me with 4 dirty rags. I didn't use any extra water to rinse out the rags, but added them to the cleaning rag pile from house-cleaning that will be washed with bath towels this afternoon.
Despite the huge water savings using a bucket instead of a hose, there are circumstances where a hose would be very useful. The most obvious to me is if you live in an area that uses salt on the roads in winter. To protect the underside of a car, you'd want to use your hose to rinse the undercarriage at the end of the winter season. However, using a bucket and rags for the remainder of the car would save additional water.
The cost of water keeps rising in my area. You'd think living where it rains a lot that we'd have low water bills. We don't. When averaged out over the course of a year, our water bill is our highest utility bill, higher even than heat. Our supplier recently switched to a tiered-use system of billing, meaning that over a certain amount of use, we are billed at a higher rate. That hits families and large households hard, as naturally, the more people using water on one bill would result in a greater water usage. To compensate, our family is very careful with water use, asking everyone to take shorter showers, running the dishwasher and washing machine only when there is a full load, using low-flush toilets, allowing our lawn to go brown in summer, and using rain water and kitchen waste water to water our gardens. I want my car to look clean, but I don't want to use so much extra water that it pushes our month's use into the next tier. For this summer, I'm choosing a bucket.
