What is the Weight Volvo 780?

To give an accurate answer, you have to give us an idea of how the truck is configured.. a 780 with standard 12k steer and 40k drive axles is going to differ greatly in weight than a heavy haul truck with, say, a 20k steer and 50k drive axles. The motor is going to affect weight, as well. The lightest motor will be the Volvo VE D 12, and the heaviest will be the Volvo VE D 16. In between those motors, there were several other motors available in various years - 12.7 litre 60 Series Detroit, Cummins M11, N14, and ISX, Caterpillar 3306, 3406, C12, C13, C15, and possibly the C16, etc...