I never did figure out this overdrive thing as in the example that i have observed, it was just a con job.
In Australia, the 910 Bluebird model was available with the four speed, direct-in-fourth trans, along with it's 3.7 to 1 diff, or the five speed overdrive box with a 3.9 to 1 ratio diff.
I have not done the sums but i suspect that the final overall ratio is the same, or very close to it. If this is true, then the five speed OD is a pain with it's detented top gear position.
In this illustration, i believe that a direct-in-fifth trans with a 3.7 to 1 diff would be a better combination.
For basic ho-hum driving, the Bluebird OD five speed in my 200B [810] is just fine as it becomes a true overdrive. The first three gears have almost identical ratio's to the four speed that it replaced. [4th is still 1 to 1 in both]
The dog leg design with a 1 to 1 fifth is a real fun box to get "sporty" with.
"Rally" in my name? My rally driving skills are extrordinary. Extrordinarily inept that is. I will stick to what i know best, & thats strictly street.