[unruhjonny]

@ OPH, thank you very much for speaking up with such a detailed response.

What do you mean by "STT" - I am guessing this is meant to indicate the (standard) Muncie three speed?

I actually created this thread here because it has appeared as though exponentially more GTO's and Tempest/Lemans were built with the Muncie three speed (offered through to 1974 in the A-body) than the Firebirds built with the three speed (only offered 1970 & 1971).

Originally Posted by 'ol Pinion head

Very very few '71 Pontiac A-bodys were ordered with 3.90 gearing. Have records on over 1/6 of all '71 & 71 1/2 GT-37's produced, & only 2 400 4spd cars have shown up with 3.90 (opt ratio rear) with the HD 4spd (M22). Have noted invoice copies on three 71 GTO's factory equipped with 3.90 HD STT.

First I didn't know that 3.90's were actually available in 1971, but only five(!!) cars being recorded as having the option, makes it seem incredibly rare!!
I did not expect that.

Originally Posted by 'ol Pinion head

No factory 3.90 gears for the new for '71 model 8.5 F-body axle.

yes, my understanding is that this ratio was never offered outside of the "BOP" 10 bolt rear, never offered for either 1970 or 1971 on the Firebird;
The closest was 3.73 offered both years, on both the 12 bolt (1970) and 10 bolt (1971) "corporate" rear end.
I believe that after 1971, the best ratio you could ever get in a Firebird from the factory was 3.42:1.

Originally Posted by 'ol Pinion head

Have never ran across evidence of anything lower ratio than a 3.55 STT factory installed behind the LS5 engine in a '71 A-body. The 71 GM service manual was printed way before actual production. It has a listing & for years there have been internet sites showing charts which list a code for a 4.33 HD STT rear in a '71 400 A-body. Didnt happen, was never actually offered.

Interesting on both parts, thanks for telling me this.
Maybe I'm wrong, but it has seemed to me that 3.73:1 wasn't an incredibly uncommon option for 1971 LS5 Formula's or Trans Am's!?

Originally Posted by 'ol Pinion head

For '72 models 3.55 was as low a ratio as was offered in A-body's, no factory optional 3.90 HD STT rear.

This is not a surprise to me;
Actually the surprise is that even 3.55 was offered - does this mean that 1972 Pontiac A-bodies used a 'corporate' 12 bolt, or a 'BOP' 10 bolt rear?

Originally Posted by 'ol Pinion head

In the magazine roadtests of '70 T/A's there is a roadtest which notes 3.90 ratio rear, whether someone at Pontiac Engr set up a 3.90 ratio 12 bolt R&P in the test car's rear & the car was some kind of factory "Ringer", OR whether the magazine writer was wrong & assumed the tested 70 T/A had a 3.90 ratio rear, is all up in the air.

I had always assumed the writer made a mistake, and assumed that the 1970 Firebird RAIV standard gear ratio was the same as 1969 - but have understood that the different design rear dictated the ratio change.
I have understood that 3.90:1 was never offered for the 12 bolt rear, and that is only a more recent aftermarket offering.
I will be quick to admit, I am not too up to speed on 1970-1972 GTO/Lemans ect equipment & options.

Again, I only chose to point out this ratio because it was what was used for 1968 RA Firebirds, and was still shown as available for 1970 Pontiac A-bodies with the Muncie three speed (see attached picture in post #1).