Having built the largest mobile car wash business on the Planet and the most efficient systems in the industry over a 27-year period before retiring, folks still to this day email me and ask me questions about the industry. Not long ago someone emailed me claiming to be interested in setting up a mobile car wash business and he asked the following questions:
"How many gallons of water does one wash use?"
"How many gallons a day does a mobile car wash use?"
"How many cars can you do in an 8 hour day?"
Well, for our systems, a really dirty SUV might use 7-10 gallons max, once you get good at it 5-6. Cars between 3-5 gallons, and once you get good about 2.3 gallons per car. This is because pressure washers only put out about (5.5 hp at 1800 psi) 2.3 gallons per minute (20-seconds spray down, 40 seconds of rinse). How many gallons per day, figure a crew of two on a busy day 60-car washes, or 80-100 if you do NO Wax jobs, which hardly ever happens.
A 200-gallon loaf tank is plenty, I recommend that you use a 110-gallon loaf tank if you have a pickup and put it in the back, if that truck is less than a 1-ton. Also this requires a little more patience when driving.
You need to know that this business is not an 8-hour yuppie business day type of industry. You work when the customer needs you. You might get up early one day for rental cars, work late another day cleaning school buses up until 10 PM (BusWash.com) and some customers will demand weekends, fleets mostly, including Sunday if you want to get it all done on time.
If you are a wimp about working more than 8-hours, get a job, join the union, and forget this industry all together. Sorry, for being blunt, but if you don't have what it takes you have no business being in this industry, it will eat you alive, take all your money too.