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Can a '65 GTO originally equipped with the Safe-T-Track rear end have been originally mated with a Muncie M-20? The reason i ask is that the restoration guide states that the Safe-T-Track was mandatory with the close ratio Muncie. Am i correct then that since the M-20 is a wide ratio, and therefore not correct for a goat with the posi? Thanks.
 

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The M-20 was available with safe-t - Track. It was an option The reason it was mandatory with the M-21 is that the gear ratio for an M-21 was 3.90 and 4.33 (at least for 68). My 68 has a 3.55 4 pinion rear with the m-20.
 

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My '65 came with an M-20 and a 3.55 safe-t-track from the factory. I was not aware of a mandatory safe-t-track with the M21 and 3.90-up gears. In fact, I've heard of several GTO's equipped with the M21, 3.90 gear, and an open rear end. Let the confusion begin! That said, of the many early GTO's I've dealt with over the years, the M-20/3.55 safe-t-track was a pretty common combination. I've owned 3 or 4 goats equipped that way.
 

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Discussion Starter #4
Thanks for the responses and helping to clear that up! On the mandatory part, which caused my confusion is, and i quote (this in reference to 1964-65): "The Safe-T-Track limited slip differential was mandatory with the close-ratio, as were heavy duty fan and metallic brake linings (RPO 692), in both years. The 4.33:1 ratio was available only as a dealer installed-installed option in 1964, but became a factory item in 1965". This source is from the Restoration Guide.
 

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The '67 GTO I owned had the close ratio Muncie and 3.90 non-posi, and the factory frame stiffeners. I swapped the rear out early on with another person who had a '65 389/automatic and highway gearing so I could get much better gas mileage -it was probably a 3.08 as I would burn up new clutches fairly quickly. The other person blew up the rear end doing a roll-back burn out by dropping it into gear. He put a pinion gear right through the bottom of the housing which left a nice hole in the bottom. I got the rear end back rather than scrap it at the time. I did pull the cover off to take a closer look at what happened. You could see it was definitely not a posi unit. It may have been a 4-pinion, but I would have not known much about that at the time. I eventually gave it away.
 

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This is a good subject. In 1965 there were three 4 speeds to choose from.

M20 (wide ratio) used with all factory available axle ratios except 3.90. (code W).

M20 (wide ratio)
wide ratio used when 3.90 rear axle was special ordered (code 9).

M21 (close ratio) (code 8) could be ordered on the All Series Special Equipment Order Form (sales code 778) but was only available with GTO, Tri-Power, and 3.90 axle (you were not required to order the M21 to get the 3.90 axle).

What the original poster was reading is about the sales code 778. This is the M21 trans and it requires options 432 (HD radiator), 692 (metallic brake linings), 701 (safe-t-track), and the 3.90 ratio, with the tri-pwr GTO option.

You didn't have to order Sales Code 778 to get the rest of those options. If you checked off the 4 spd option on the regular Order Form and listed the 3.90 axle on the All Series Special Equipment Order Form, Pontiac selected and installed the code 9 trans (the only difference between it and the code W is the internal speedo gear and neither will have the groove ring because both were wide ratio). In this option you could choose not to have safe-t-track.
 

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The M-21 close ratio trans is the most misunderstood option on the 1964/1965 GTOs.

Most believe that if you special ordered the 3.90 axle, that you received the M21 close ratio trans. This is not correct. In this case you received the wide ratio M-20 code 9.

If you special ordered the M-21 close ratio (code 8) then you automatically received the 3.90 axle along with other options I described in the previous post. The key was to special order the trans and not special order the 3.90 axle.

I can't tell you how many people believe they have a M-21 with their 3.90 gears and when you look at their PHS billing history card it say code 9 in box number 77, which means it is NOT a close ratio.
 

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And I'll add this: about 90% of the people I talk to assume that their Muncie IS an "M-21" and that it is a stronger, superior gearbox to the M-20. I'm talking every GM make with every rear end ratio. "Yeah, it's got an M-21 and a posi." The M20 is the better gearbox for any rear ration numerically smaller than 3.90. Much better ratios.
 

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When I was special ordering my 65 GTO, I looked into the M-21 option. I pondered over this option and multiplied the close ratio gears to the final drive 3.90 ratio and compared it to the wide ratio gears to the final drive of the 3.55 ratio. At least on paper the wide ratio seems to have better pulling numbers through 3rd gear, so I went with the wide ratio transmission.
 

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Roger, when you were specai ordering your '65 GTO, I was tearing up the sidewalks with my Murray Fireball 500 pedal car (with custom exhaust pipes, I might add!)!! LOL
 
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