I am going to tell the "long version" of my 4-speed/OD story because I learned a lot that I think would be useful to share. I will break it up into a series of "bite-size" posts. WARNING: Do not subscribe to this thread if you are easily bored.

Overdrives had a mystique about them for over half a century. They are associated with combining good economy with performance. In pure mechanical analysis it is arguable that a straight trans with more gears than a three-speed should give an equal combination of economy and performance if it had the proper ratios. It would be simpler and perhaps a little more efficient if the top gear was direct, with no gear losses. From a assembly-line point of view, the three-speed overdrive transmission was attractive because it could be an extra-cost option that would more or less directly replace a three-speed, using the standard column shift.

With a normal three-speed, the overdrive usually only offers a fourth top gear for the highway. First OD and second OD are rarely used and are usually not an even split of the gears. When applied to a close-ratio four-speed, the only additional ratio is the top gear - all the others are redundant. Still, in 1968, the idea of an overdrive on my T-10 was attractive. After enjoying the small rpm drops on my Isky-cammed 312, in a ‘56 Vicky, I decided that the only way to get anywhere on the highway with a 4.56 9” Ford rear end was to put an overdrive on the back of the T-10. I am going to discuss this trans before the T-98/T-85OD because there are a lot of useful “lessons learned”.

Feel free to add thoughts or disagreements - this is a conversation.