Well when I wash my car without a hose...5 gallons. A 5gal bucket ~3/4 full, and a 1 gal rinse bucket.
As far as washing with a hose...some people just let the water run while they wash their car, some have leaky hoses, some rinse the car more/better than others. IMO it's impossible to estimate how much water is used to wash a car.
The rinse bucket is to rinse off the MF(s) that I'm using to wash the car. I rinse them in the rinse bucket so that I don't leave all that dirt and crap in the wash bucket to be picked up later. I have a Grit Guard at the bottom of the 5gal bucket as a precaution, but I still try to keep as much dirt as possible out of the wash bucket.
I don't need to rinse the car when washing without the hose.
I've been using QEW all fall and winter and have that method pretty well down pat.
But I've never been comfortable about using the QEW method on a heavily road salted car. So all winter I've been hittng a Laser Wash and using the pressure gun there fior the wheels and all at least once a week.
That gets expensive and the lines are always long.
I'm also looking foward to the spring when I do my full detail on the car. I just don't see QEW as my jump off point for that.
I use and love QEW and am not knocking it. But for me it's another tool in the arsenal.
Anywho I can get a 25 gal polyethylene tank for about 50 bucks. Combine that with a 1-1/2 hp self-priming pump and I'm in business.