How are you calculating it? By computer or by hand? The computer is typically in the ballpark for me but it's still off by varying degrees when I compare with calculating at the pump.
I'd say it's worth trying out the old filter for a tank and switching back to see if there's an obvious difference since I'm wondering if the increased airflow makes any difference for your car.
So I do the math.
I go by my trip mileage ÷ Gallons to fill up = avg mpg
I need to put gas today, the gas light came on. I'll post my exact mileage later today.
And yes, I think I need to put the original filter back in.....for science. Lol