Why is my car taking less engine oil to fill from an oil change?

I have a 2013 economy sedan, 70k miles. Fuel mileage has been a lot less than normal (32-34 combined to now 23-25). I just changed the engine oil for the second time since this has started happening. Manufacturer s book says it should take 3.8 quarts to fill. Last time that was too much so had to take some out:... show more I have a 2013 economy sedan, 70k miles. Fuel mileage has been a lot less than normal (32-34 combined to now 23-25). I just changed the engine oil for the second time since this has started happening. Manufacturer s book says it should take 3.8 quarts to fill. Last time that was too much so had to take some out: this time I only put 3.5 quarts and it was still showing too high on the dip stick so again had to take oil out. I have always changed my own oil so I know this is unusual. Spark plugs have been checked and air filter (k&n) also fine.
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