Is that true?

As with so many eco-questions, the answer is: It depends. If your idea of a fun summer afternoon involves spending hours lathering up your car and then frolicking in the hose spray, then yes, you'll probably use more water buffing your wheels than a well-managed carwash would. On the other hand, you could probably beat the pros if you committed to a super-efficient washing regimen using a bucket, a sponge and a little elbow grease.

Hose habits are the biggest variable. A common five-eighths-inch garden hose, operating with a typical water pressure of 40 pounds per square inch, spews about 11 gallons per minute. Spend just 10 minutes rinsing your car, then, and you've used about as much water as you would doing seven loads in a dishwasher -- and that's not counting any water you've used to mix up your cleanser of choice.

The pros generally stack up quite favorably against those numbers. Carwashes come in three basic flavors: in-bay automatics, in which the car remains stationary; conveyors, in which the car moves through a washing tunnel; and self-serve stations, in which customers use coin-operated devices such as spray guns and foam brushes. A 2002 study commissioned by the International Carwash Association analyzed about a dozen of each type among three cities. In-bay automatics were the thirstiest, using 17 to 69 gallons of fresh water per vehicle, though one outlier averaged 111.5 gallons. The conveyor systems studied performed similarly, using 19 to 45 gallons per vehicle, though the authors reported that up to 60 gallons wouldn't be unusual.

The self-serve stations, in turn, used just 12 to 18 gallons per vehicle. You might be able to come close to that at home if you were extremely dedicated; for example, using a towel dipped in a bucket of water to scrub off most of the grime and reserving the hose for quick rinses. It would also help to invest in one of those hand-held nozzles that allow you to start and stop the spray as needed, so that water isn't gushing out continuously. (A pressure washer is also an option: According to ConsumerSearch.com, these use about one-fifth as much water as a garden hose to accomplish the same cleaning task. Of course the trade-off is that it takes fuel to operate them.)

If you decide to go to a commercial carwash, look for one that recycles its wash water. Depending on how efficient the system is, anywhere from 10 to 80 percent of the water can be reused. And check to see if your water utility has a certification program for water-conscious carwashes, as San Antonio does.

But water use is only one factor in this equation. What happens to the dirty runoff also matters a great deal. When you wash your car in your driveway, the discharge generally flows into a storm gutter and then straight into a river, stream or bay, carrying with it not only the cleaning solution you've used but also all the oil, grease, heavy metals and other pollutants that had been clinging to your car.

Under the Clean Water Act, carwashes have to follow guidelines when collecting and disposing of their dirty wash water. Most send their runoff to municipal wastewater plants after pretreating it to remove oil, as well as grit and other sediments. So if your car needs a heavy-duty cleaning -- one with the potential to release a lot of gunk -- going to a professional makes a lot of eco-sense.

Look for companies that have phased out the use of toxic ammonium bifluoride and hydrofluoric-acid-based cleaning solutions in favor of phosphate-free, biodegradable detergents. The well-regarded EcoLogo program has certified a number of environmentally preferable products for use in commercial facilities; ask your local carwash to stock them.

Washing at home means saving energy, though, because there's no equipment to power or water to heat. So for a simple exterior spit-and-polish, home might be the greener option if you take precautions. Park in a place where runoff to storm drains can be minimized, such as on grass or gravel. Keep as much dirty water in the bucket as possible, and pour it all down your sink or toilet. The third-party Green Seal program has one carwash product on its list of certified household cleaners; in general, you should be looking for phosphate-free soaps that are readily biodegradable. And though the Lantern can't vouch for how well it will work on your Prius, you could try using a green boat cleaner, like the ones certified by EcoLogo, that's designed to be safer when going straight into waterways. Waterless carwash products are also an option, though they may not be powerful enough for truly grubby cars. Look for ones that are labeled "low VOC," meaning they emit fewer polluting volatile organic compounds.