I am nowhere near a "mechanical" guy. Dave Barry once said of himself what I often think about myself: "I'm the kind of guy who, if the 'check engine' light comes on will fix it by putting duct tape over it." That's very much like my level of mechanical know-how.
So, I do not understand how today's tip works, I just know it does. When Possible, Use Overdrive Gears. Obviously, this does not mean to use overdrive when you are traveling at 25 mph going to the grocery store. But, when you are going highway speed (especially on the interstate), this is a great way to gain a few mpg.