I've been thinking along these lines a bit myself lately, but not about adding an OD exactly...

I've got this one pretty long stretch of road that I travel twice a day, with a 45 MPH speed limit (enforced!) and I can't decide which is better, stay in 4th and let the engine run a bit closer to it's peak power range, or shift to fifth and drop the RPM . At some point I would think that you would start losing mileage the higher you gear it. But, where is that point ? How do I find it... I can see on my vacuum gauge that staying in 4th appears to be better, cuz it stays around 15" the whole time. In 5th, vacuum is usually MUCH lower.

I don't _think_ I'm hi-jacking, cuz the same principles should apply, no ?