It sound counter intuitive that a professional car wash will use less water to wash your car then if you do it yourself, but it is actually true.
Businesses have an incentive to save every penny to be profitable. They have a set amount of water that is used per car and they usually filter and recycle the water in closed loop system (ask your local car was if they have such a system, and if they don't go to one that does).
If you wash your car yourself in the driveway you will use as much as 60 gallons of water every 10 minutes your hose is used. At a car wash amount of water used per car is lower than 40 gallons.
Arizona Department of Water Resources has a guideline for car washes that outlines the amounts of water that different types of car washes use.