Option Code question for transmission
 

I have a C20 with manual transmission. It was originally option code 1MM4. Does anyone know what transmission this is? I'm assuming it's a muncie, but I want to make sure.

Thanks!

Re: Option Code question for transmission
 

Most likely it'd be a sm465

Re: Option Code question for transmission
 

Correct.

"MM4" is the "Merchandised" trans option code for "4 speed manual trans provisions". This is the code the dealer and/or customer would select, indicating a 4 speed manual transmission was desired. (By the way, the "1" is a dummy character; just a placeholder).

That code is then translated into the "non merchandised" code by the factory. Based on other powertrain considerations (model, series, engine, transmission availability and rear axle) the specific transmission type is selected.

In your case it was likely the M20 version which is the floor mounted 4 speed with the creeper gear.

Another example: if it was available and you had chosen "MM3" that would indicate a 3 speed manual trans. The other powertrain conditions would drive you into a Saginaw three speed, column shifted, with either a numerically low first gear (M62 = 2.85 first gear) or a higher first gear (M64 = 3.50 first gear).

I've attached the Powertrain chart for an '85 C20 (as found in the GM Heritage Center archives) so you can see what options were available in your specific case.

K

Re: Option Code question for transmission
 

Thank you! This is awesome. I knew I had the 3.73 gear but this really clears up the rest.