Can't speak for everyone, but my 6 and my 7 both vary about 1/4 inch on the guage in the mountains, at about half on climbs and quarter on coasting downhill. I presume because of the work load. In heavy traffic, the 7 climbs to 3/4, then the aux fan kicks on. I have not been concerned by these variations. Economy is rather good for age and the way I drive and the cars both run great.