I read something here once, but can't find it, now. I have a '99 WJ with the 4.7 V-8. I'm not sure when to use the O/D. I don't tow anything ( I may someday, just don't now), and most driving is only a few miles back/forth to work but it does include highway time. Is it preferable to use the O/D on a regular basis, or only when towing loads? I know nothing about what it's function is other than when I turn it on, my rpms go up. Does it effect the already crazy-bad gas mileage (12-16 highway, 10-12 city)?