When I switched to thte NGK coppers, I got about 1-2 MPG better. I checked this using the cruise control for all driving(at the time about 98% freeway) and keeping my foot out of it. It is a matter of having to change them more often, which I don't mind. Cu is a better conductor then Pt, which is better than Ir. I would imagine silver would be the best, but who wants to pay for that?