I'll assume you have Schedule 40 pipe. 2.5 inch has an ID of 2.469 inches, and an area of 4.7878 in^2. The volume of 69" of that (5.75 ft) is 330.4 in^3. One gallon is 231 in^3, so it will contain 1.430 gallons if full.
you would need 3 feet of pipe
61,101.8 gallons per 2,600 feet of 24" pipe.
The volume is 31.809 cubic feet.
A two inch pipe can hold 0.1632 gallons per foot. It takes slightly over 6 feet of two inch pipe to hold one gallon of water.
0.2 gallon or 25.6 fluid ounces.
0.3672 gallon per foot of pipe.
Answering "How many Cubic feet are in a 5.5 inch pipe inside 50 feet long?"
A 100-foot pipe with a 6-inch diameter has a volume of: 19.63 cubic feet.
This pipe has a volume of 24.5 cubic feet.
5.5 gallons per 15 feet of 3-inch pipe.
You would need 2,451 feet of 2-inch pipe for 400 gallons.
A 1.5 inch (diameter) pipe that is 10 feet long can hold approximately 3.32 gallons of water. This calculation is based on the volume of a cylindrical pipe.