Just replaced our MAf on a run of about 7 miles - A6 50-60mph mpg has jumped from 37 to about 43, at motorway speeds its made less difference mind you the only run I've done is along the a14 and was plagued by the usual hgv syndrome of lorries waiting til I've pulled out to overtake when they decide that they can overtake the hgv ahead - just maybe - so slow down for 3 miles inching past tanker, then accelerating past until we get to next pair of hgv's - it happened about 5 times over the 30 mile stretch so it was difficult to stay on cruise.