Previously, with a full tune-up and all fluids changed, about 90% highway driving, I was consistently getting 15.3 MPG with my 1998 Ford Expedition Eddie Bauer (5.4L 4WD). The ONLY thing I changed was poking this thing into the airbox. As a result, I'm now averaging 16.6 MPG driving exactly the same route under the same conditions with the same fuel. Can't really tell if it added any horsepower or not - it's a big ol' honkin' SUV, not a sports car - but it'll pay for itself in fuel savings, alone, with the amount of mileage I drive every day. Definitely a worthwhile purchase.