On the day I got my 3spd with OD running, I was playing/experimenting with the shift patterns for final tweaks of the shift linkage.
I made a run from Bremerton to Tacoma, Washington. I was well down the road and just starting to be unimpressed with the gain when it dawned on me I was still in 2nd over, which wasn't far off of what 3rd, direct, with the other transmissions had been. I went to third, then OD and that 30% drop made it all worth while. The tac dropped to around 2,000 (before, 70 mph pushed it the engine near the 3,000 mark) and the headers got quiet.
I thought the OD was such a good investment, had it gone south, I was looking at things like the T-10, or Hone overdrives .
You had to knock about 15" off your drive line to run them, but they'd run on anything (auto or stick, including 4-wheel drives, IF you could take it out of four wheel (some silly thing about having your front tires turn 32% slower than the back ones)).
Back in the sixties and seventies, a lot of people bought the overdrives, then installed them in their motor homes. The OD's didn't like that, so Hone made a tougher version to handle up to 10,000 pounds. Thought it was the T10, but that was fifty years ago, so . . . .