Every inch of height in your tank equals 2.49 gallons of water. So 2.5 inches in height is equal to 6.23 gallons, and 3 inches of height is actually equal to 7.47 gallons. You can play Archimedes and fill your tank high enough to cover your driftwood and mark the height of the waterline. Place the driftwood in the tank and mark the new waterline. The distance between the two marks will allow you to calculate the actual amount of water the wood displaces.
Go on, play scientist and let us know how you do!