What's said to be the world's biggest truck wash is located in Lloydminster, Canada. It apparently consumes almost 20,000 US gallons of water (about 75,000 liters) to wash a single truck!
The undercarriage wash alone (i.e. wheels, suspension and frame) uses almost 1,500 gallons each minute. I guess that's necessary to get rid of the salt, slush, snow and ice from winter roads, or the dust, mud and grime of summer runs. It cost $140 (Canadian) to run the truck shown above through the wash.