Overdrive Options

Guys I am totally obsessed with an overdrive unit in my Dart. So much so I might shoot for teaching, I'm a high school teacher, summer school just for an overdrive option. For instance just right now on the highway all I could think about was "lower rpm's...lower rpm's..lower rpm's..".
First time I drove the Fairlane with the t-5 in it I was hooked. When I was driving it on a somewhat daily basis every now & then I would leave it in 4th on the freeway, for what seemed to be the entire 15 or so miles home. In reality it was tenths of a mile. You will see a lot of folks saying that you will never recoup the money it takes to put an od in your car because your mileage will only go up x.x miles per gallon. While this may be true the car will be a lot more enjoyable to drive......what is that worth?