Tanker trucks are vehicles with large tanks for transporting liquids. They are designed for specific cargoes and service requirements, and are built in a range of sizes. Tanker trucks are equipped with a pumping system for loading and off-loading liquids.
Tankers are classified as tank trailers and trucks (no trailer). Tank trailers are further classified as: semi, a tractor and a trailer; double, a tractor and two trailers; and trailer, a truck with a tank body pulling a smaller trailer.
Semi-trailer is the tanker configuration with the largest load-carrying capacity, up to 9,100 gallons (34,447 liters). The other trailer configurations, being somewhat heavier, can carry less and have capacities up to 8,900 gallons (33,690 liters).
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U.S. government sources make no clear distinction between large and small tankers; the U.S. Environmental Protection Agency noting, for example, that “tank truck” is a term requiring clarification. Various other sources distinguish small tankers as those with load capacities less than 2,000 to 3,500 U.S. gallons.