Number of gallons in a shipping trucks tank?

Depends on the load (the product), and the empty tractor-trailer's weight.

Typically, the empty tractor trailer will weigh between 25,000 and 26,000 lbs. The maximum gross weight must be 80,000 lbs., or less. Let's assume a tare weight of 25K. That means we can take on 55,000 lbs of product. If our product is, say, methanol - which weighs 6.63 lbs. per gallon - we can take on about 8200 gallons. In practice, it's usually 8000 to 8100 gallons.

If the product were 50% sodium hydroxide - which weighs about 12.76 lbs per gallon - we would end up with about 4300 gallons.

Gasoline is about the same weight as methanol. Any tanker truck will almost always try to take on the maximum legal weight of 80,000 lbs. By comparison, the average weight of a car is around 4,000 lbs.