Gasoline Octane -what is the difference ?

['75-4x4guy]

This sentence is right out of my 1974 owners manual

" The engine in your truck is designed to operate on "unleaded", "low lead",or "regular" gasoline with a research octane rating of at least 91 when the engine is adjusted to factory recommended specifications and does not require "premium" fuel."
My question: Is gasoline rated the same (today) as it was in 1974 ?
If it is different then how ?

[68F250]

No it's not the same rating. Basically there are three ratings, the Research method, the Motor method, and the average of the two. The average is the rating you see on the pumps (R+M)/2. Here's some good info on octane:

http://www.prime-mover.org/Engines/GArt ... ctane.html