So I'm fairly new to the forum but have had the jeep for a little bit and have been playing around with it to see what she likes best. Lately I've been trying out turning O/D off to see how she runs when I go back and forth to work in the city (at or below 55km/h or 35mph the whole time). I've been into cars for a while and even as a kid talked to my dad all the time when we were towing our trailer asking him questions and stuff. Also, I get a little bored driving an auto so I tend to start pushing buttons

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Anyways, I've read a lot of posts saying to only turn O/D off if you are towing...which is true, though there are a few (wise) people who say they turn it off around town. Some people seem to think that you will kill your mpg if you do so because you are essentially eliminating a gear. BUT, I can vouch for the fact that in my '02 liberty 4x4 auto that I went from around 16-18mpg with O/D on, to 22-23.5mpg with it turned off...on fairly flat ground, with 4-5 stops and starts on my route. Now this could be a result of my route so it may not be the same for you...
Moral of the story, if you are driving around town, a fairly significant fuel savings can be had from simply pressing that little button on your shifter...no money spent on fancy/worthless fuel economy gadgets. This makes sense too because your tranny won't be switching between gears as much, and you are driving around closer to the RPM "sweet spot". I also took the time this morning to figure out that it will save me ~$5-700/year if I do this...depending on how heavy my foot is some days....