The answer to this riddle is "Washington." When a car is dirty, you would "wash" it. Removing the "r" from "ring" gives you the "ing for the middle of the word, and "ton" is the unit of measure that describes 2,000 pounds.
This riddle only works in the parts of the world that don't use the metric system as a standard unit of measure. For example, in the United Kingdom, a ton is 2,240 pounds (known in the United States as a "metric ton"). To distinguish between the two, 2,000 pounds is sometimes referred to as the "short ton" and 2,240 pounds is the "long ton."