Ever wonder which is the best way to save on fuel costs? There are three approaches: 1.Reduce gross vehicle weight 2.Improve aerodynamics 3.Slow down. Reducing weight has no appreciable effect on mpg once you reach cruise speed because weight only affects energy requirements on hills and when changing velocity (inertia changes), usually a minor part of any trip. Aerodynamics can only be changed (by owners) with wind deflectors and ground effects packages and the result is nil., approx. 0.2 mpg or so. Manufacturers could improve aerodynamic design, but at a significant increase in rv length. Slowing down has a huge effect on mpg. This is because wind load goes up as the cube of velocity. It is because to move forward, you have to lift a column of air (5 miles thick), or push it sideways to get through. At low velocities air is easy to push sideways (a linear function), however at high velocity the equation becomes non-linear due to the mechanics of compressibility of gases. At high velocity, you have to lift the column, because you cannot effictively push it around you. Try opening your door at 70 mph. Multiply 14.7 pounds per square inch by the area of your vehicle's hood and windshield (in sq. in.) You are lifting tons of air per mile and the more miles per hour = more tons per hour. It gets nasty fast. Going from 55 mph to 75 mph is approx. a 36% increase in velocity, however it is a 250% increase in wind load. This can result in a decline of approx. 40-50% in mpg in most vehicles. I tested this on several class A's and found that at 55 mpg versus 75 mpg, the mileage average changed from 10-11 mpg down to 5-6 mpg. The effect is about one third on diesels due to their torque/hp curve. Slowing down also has a significant effect on tire and brake life, not to mention drivetrain wear. Safety goes without saying. The downside however is that it takes a little longer to get somewhere (not a problem if you're retired or fired). Also a lot of people get mad if you are poking along. I just turn up the radio. Just an interesting observation by a old worn out engineer. Hope this helps someone.