"Laziness", CelloDad is fond of saying, "is the mother of invention". CelloDad is one of the most inventive people I have ever met. I have learned many tricks from him over the years we have been together.
One effective way to get things done is to hold back the urge to do something, but instead to simply wait until the problem goes away. This approach works particularly well for dusty cars. Is your car covered in mud, or liberally dusted in tree pollen? Take heart: if you wait long enough, a good thunderstorm will eventually visit your parking spot, the heavens will open, and torrential rain will wash away all that dirt and dust, and leave your car sparkling.
I have applied this tactic many times, to great effect, and with satisfyingly little effort.
It saves you from releasing nasty chemicals, that might lurk in your car soap or waxing finish, into the environment. And it saves a lot of water. It also saves you a lot of work of the kind that requires you to take an extra shower: more water savings.
But once in a while you can't wait for the saving rain. Perhaps you need your car for a big date, before the dry season is over. Perhaps you need to get the winter road salt off your wheels before they start to rust (because when you traded in the expensive and dent-prone supersized wheels, your car dealer got you back by delivering steel wheels not aluminium, together with the fat rebate check). Perhaps you finally get too irked by the dust stuck under the overhang that now mars the streamline of too many car posteriors: perfect for trapping dust, and for preventing the rain from washing it off.
In those unfortunate cases, you can't get around it: you have to actively do something to get the car clean. So let us go over the choices, in order of decreasing work on your part.
Option 1: Hand washing, at home
My dad had a neighbour who washed his car every weekend. Unless it was pouring rain, he would be out there with his sponges and his cloths and his various lotions and potions for the car. Since he lived on the 6th floor of an apartment building, and there was no water faucet at street level, he would come downstairs carrying two buckets: one with his soapy solution, and one with water. He would lather one part of his car, rinse it sparingly with a second sponge, rub it dry with a cloth, and move on to the next section. He probably used less than 5 gallons of water for the whole wash.
Those of us who don't live in apartment buildings find it easier to use a garden hose for rinsing the car: it's a lot less work. If you've done a good enough job on the lathering and the rinsing, you can air-dry the car without getting too many streaks, especially if you choose a cloudy day.
But it does take more water than two buckets: 30 gallons if you have a shut-off nozzle on your garden hose, and a whopping 100 gallons if you let the hose run during your carwashing routine. This is the reason that car wash bans are imposed during periods of drought: such a ban can really save a substantial amount of water.
Not only that, the soap, the wax carrier and the dirt coming off your car (which includes soot, particles from your brake pads, oily residues from here and there on the engine and other nasty chemicals from general road dirt) can wash off your driveway and into the storm drain on the street, from where it will go into streams and rivers, and eventually back to you in the form of your drinking water. (That's why they call it the water cycle: what goes around comes around). I am not making this up: pharmaceuticals and other synthetic chemicals that have made their way into the sewer system have been detected in city drinking water.
So if you must wash your car at home, put it on the lawn: there the run-off will be absorbed by the turf and filtered by the soil before reaching the aquifer below. You don't want your grass to drink all that dirt? well, why should the aquatic life? Hmm - Perhaps doing it at home is not the cleanest overall option. Besides, it requires the largest amount of work. Enter the car wash.
Option 2: Self-serve car wash
This is a row of bays fitted with various hoses with nozzles: a water spray for pre-soak, a lather of suds, a foam brush, a rinse and optionally a waxing step. They cost $2-5 per wash, mostly because it's you doing all the work, and all while moving at the pace dictated by the car wash program.
Italian self-serve car wash
But the whole thing uses about 15 gallons of water. This is because the rinse stage uses what's essentially a power washer which pushes the suds off the car rather than rinsing it gently. So you want to be careful about not getting it too close to the nice paint on your car. You also want to wear rain gear while doing this, especially in an enclosed bay. The demos invariably show well-dressed people who manage to preserve their hairdo while doing this, but I have never wielded a power washer without getting soaked.
Between that and the sweat your worked up darting around your car trying to cover it all before you run out of suds or water, you'll be ready for a shower after the car wash: so if you do an honest reckoning, the shower water needs to be added to the 15 gallons of the car wash itself.
Option 3: Full-serve car wash
These come in two types: one is in-bay, very much like the self-serve car wash except that you're paying someone else to do the work. The other is a conveyor-type car wash where your car gets pulled past the various stations, mostly automated except for a manual washing of the wheels and wheel wells before, and a cloth finish after the tunnel. The cost starts at around $10 for a basic wash, and can run a whole lot higher than that, depending on the options you want to tack on.
But even an in-bay wash can use 35 gallons of water, and the water use can go as high as 100 gallons: same as letting your garden hose run during a home car wash. However, the good news on this is that many car wash places recycle their water after filtering it. And the waste water goes into the sewer system where it is treated (before release into streams; you just hope that that treatment is good enough). Ask your local car wash if they recycle their water.
My conclusion: the full-serve car wash that recycles water does a really good job of getting your car clean, with rather minimal water use. Without any work on your part. Even a good thunderstorm can't beat that.
Of course, a thunderstorm is free.