You mean it spoils?ONR's great, but doesn't sit well imo. Supposedly stills works the same, but loses the color tint and smells.
I'm not really looking to save water, but my normal method requires 16 gallons in buckets + untold gallons of rinse water from the hose. I use the two-bucket method, and I don't use wheel water after the wheels are done, so that doubles it. Grit guards take up the first gallon, so a minimum of two per bucket are needed. I would never save wash water for another use unless I was doing cars back to back (which almost never happens).I use a LOT less water than you though, about 2-3 gallons and its good for 1-2 washes.