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).
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.