Why is my 2003 mustang gt v8 manual wasting so much gas? It is literally wasting about 5-10 mpg is this normal

Do you ever have a puddle of fuel under your idling vehicle? Do you have any check engine codes? How do you drive the vehicle? When did you last change the air and fuel filters?

Well no and I just bought the car I've been with it for about 3 months and I bought it from a dealer. I tend to change gears till I hit about 3 rpm could that be it because it came with a stack racing air filter

Pull a spark plug or 2 and check condition

What is your actual MPG? Fill the tank, record the miles, drive it several days, refill the tank, subtract the new mileage reading from the initial mileage reading, divide the number of miles traveled by the number of gallons used. Idling, warming up the engine, low tire pressure, high speed driving, heavy acceleration, Wide tires, air dams, wings, etc. Is the "Check Engine" light on? You can pull spark plugs all day and air and fuel filters don't effect mileage or performance. Not in a vehicle equipped with a MAF.