Self-service carwash stations limit you to around 17 or 18 gallons of water. And most full-service stations average 30 to 45 gallons of water per car, according to a 2018 study by the International Carwash Association.

Many machine carwashes recycle and reuse water, which helps their bottom line, Mr. Minchillo said. And at self-service washes, high-pressure nozzles and timers keep water use to a minimum.

But the biggest reason to go to a carwash is to prevent pollution, Mr. Minchillo said. When you wash your car at home, all the dirt, oil, engine fluids, soapy phosphates and chlorides from the soap can run with the water into a storm drain and into nearby rivers and lakes.

Carwash businesses are supposed to collect the water they use when it can no longer be recycled and send it to a wastewater treatment plant, where the pollutants can be filtered out before the water flows into the ecosystem.

State and local governments in places like Washington and New York encourage residents to take their dirty cars to a carwash to prevent this kind of runoff pollution. The Environmental Protection Agency also recommends washing your car less often and having it done by a professional when possible. If you have to wash your car yourself, the agency recommends using a bucket rather than a hose.