I think it would depend on how heavy the aircraft is before your extra 2.5t got added. It would take more fuel at near MTOW (Max Take Off Weight) to carry it than it would at a lower weight. Also how high it was going to climb to (depending on route etc) as well and the speed it was going to fly.

That is about all I can help with, but on another multi engined aircraft of that age, albeit at a much faster speed, it would cost you 250 kg (0.25 t) to carry 2.5 ton for 3 hours. But if it was less than 2t it would not be taken into account.

Surely the aircraft comes with an ODM. Surely the ODM is full of graphs for working exactly things like this out?
However, what you need is the fuel burn for a given weight. Doesn't matter if it's 2.5T less or more. There should be a published burn Vs weight graph/table somewhere.

WM

Shouldn't this thread be closed, preserved and marked 'BOI Evidence' or something?

Have internet, will work out own problems. But you could start here:

http://www.baseops.net/c130pilotgouge.html

:cool: