M-20 with L-78 or L-72 & deep gear -How?

[Paul_S]

What are the book/order form limits for '65-'69 Passenger, Chevelle, Nova and Camaro L-78 or L-72 cars with an M-20? As far I know it depends on the rear gear ratio, but what are the "rules" and are there documented exceptions? If you ordered a 4-speed in place of the standard HD 3-speed and a highway gear (say 2.73-3.31) then you were given the M-20, but if you ordered a deep gear (say 3.55-4.88) then you were given an M-21. I think the limits I listed are correct for '65-'66 passenger, but for '66 Chevelle the limit is 4.10 with M-20. Why? What are the limits for the other years/cars? One of the main things that made me ask/question this was the post of the first '67 BM Chevelle. The car originally came with an L-78, M-20 and 4.56:1 rear. Did they change the option limits for '67 Chevelles? Is M-20 listed on the original paperwork for the car? As far as I know the P.O.P. does not specify M-20 or M-21.

I'm just trying to figure out why and how things were done/ordered. If you have any info, original documentation with the option details for either car/year or think I'm off on something, please post.

Thanks, Paul