pbiggs said

Well....I have 2 1966 Formula S 4 speed cars, both w/Suregrip and both 7 1/4 from the factory. I do know that 8 3/4 rears were in some early A-bodies, but I have never figured out a customer specified and ordered one with the 8 3/4.
The tow package may have had the larger rear, but tow packages specified only available with automatic transmissions.
In 1967 the 273 hp with 4 speed came standard with the 8 3/4.
Perhaps later build 66's with manual transmission and hp engines came with the 8 3/4, but most early ones I've come across didn't.
I have also read that when a 7 1/4 went belly-up under warranty, Chrysler replaced them with 8 3/4's.
A lot of speculation here, if somebody knows the facts, I loved to hear them.

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