Read this article about the similarities and differences:
http://www.webrodder.com/article.php?AID=426As joker just said, you might not need an overdrive. Depending on your engine, camshaft, rear gears, and tire diameter, you may be lugging at highway speed. A mistake I have seen many guys make is put in an overdrive and then have to run in third gear all the time, because the overdrive drops the RPM too much at cruising speed. Either that or assuming they'll get better mileage with an overdrive, when it drops the RPM out of the range of where the camshaft wants to be running ideally, and they get worse fuel mileage. Just some things to think about.
