The "by ear" setting referenced was in quite a bit of the older American make service manuals in prior times, before automatics were in most every vehicle built.
As I mentioned a while back, when the "posted" octane numbers first came out, there was a little math equation on the bottom of the sticker (R+M) / 2. That meant that the posted octane was the average of the sum of the Research and Motor methods of determining and rating octane. Research numbers are what we all commonly knew about, but Motor numbers were much lower and what engineers more likely knew about.
For what was commonly known (by brand) to be 100 Research octane had a 95 pump octane rating. By the same information, leaded regular back then was about 95 Research octane, but became about 92 Research when most of the lead was removed.
By that same token, in the SAE Paper that Chrysler engineers presented with the introduction of the new wedge V-8, it was noted in the commentary that the engine was calibrated to 97 Research octane fuel (Premium fuel in 1957). Therefore, as there appears to be a 5 number level between Research and Posted octane numbers, that would equate to a modern 92 Posted octane number for Super Unleaded fuel. By observation, also, at the higher altitudes, you might not find 93 Posted octane fuels, but only 92 or 91 for the premium unleaded fuels.
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