These questions will have to come with all kinds of qualifiers. First of all, every state has weight limits for trucks. Twenty-six states maintain minimum federal gross vehicle weights (weight of truck and load) of 80,000 pounds, while 19 states permit trucks of 105,000 pounds or more. Michigan tops the list with 115,000 pounds. That said, overload permits can be purchased in several states, and there may be seasonal weight restrictions and/or bridge weight restrictions in rural areas.
About 80 percent of milk trucks are tractor-trailers with the remaining being straight chassis tank trucks. A typical straight chassis truck will have a 4,000 to 5,000 gallon tank, while a typical trailer will hold 7,000 to 8,000 gallons.
The final complication in these answers is based on where the trucks are coming from and how far are they going. A lot of milk is moved long distances into and out of the Southeast for seasonal balancing. States like Florida require that long-distance hauls have tractors with sleeping cabs. This additional weight on the tractor reduces the amount of milk that can be transported and still make the road weight limits on the trucks.
Mark Stephenson, Cornell Program on Dairy Markets and Policy