I have an 88 4 cyl, three speed automatic. The tranny feels like it's giving up the ghost so I am wondering if I can swap in a similar tranny from an earlier year car or not. Are they all the same or are there changes needed to make one years work with another? Not looking to do anything clever here, just swap same for same to keep costs down and do the swap as quickly as possible (car is kids daily driver so need to swap in a new tranny over a weekend, hence the need for simplicity).
According to a Fiero reference book I have here written by Gary Witzenburg, all automatic transmissions that were used in Fieros were the same and used the same ratios regardless whther they were used on the 2.5L or 2.8L engines. They're called THM125's. They were used in alot of other cars too, but they may have had other gears to compensate for differences in weight , power, etc.
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11:27 PM
IFLYR22 Member
Posts: 1775 From: Tucson, AZ. Registered: May 2007
If ya gotta put a tranny in, look into getting a 4-speed. It was used with the 2.5 and 2.8 in various GM front wheel drive models. I think the model is T-440. The fourth gear is overdrive and will boost fuel economy significantly
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01:47 AM
Oreif Member
Posts: 16460 From: Schaumburg, IL Registered: Jan 2000
According to a Fiero reference book I have here written by Gary Witzenburg, all automatic transmissions that were used in Fieros were the same and used the same ratios regardless whther they were used on the 2.5L or 2.8L engines. They're called THM125's. They were used in alot of other cars too, but they may have had other gears to compensate for differences in weight , power, etc.
No, That is not accurate. The 1984 thru 1986 had a 3.18 final drive gear ratio. The 1987 and 1988 used a 3.33 final drive gear ratio. It does not matter which engine is used. Aside from the final drive, all the auto's are the same.
You can tell which final drive you have by the RPO codes: F75 Transaxle, final drive, 3.18 ratio GX3 Transaxle, final drive, 3.33 ratio
[This message has been edited by Oreif (edited 07-27-2009).]
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
I don't doubt you Oreif, just curious what the source of your info is?
General Motors published specs in the service manuals. GM 1984-1988 GM P-Catalog list the MD9 transaxles as having 2 gear/sets: the 1984-1986 use a 33/37 (3.18) set and the 1987-1988 use a 35/35 set (3.33)
I had an '87 Buick Century with a 2.5L Duke and TH125c trans. It had the Buick final drive ratio at 2.84. It would cruise very nice at 70mph and get about 34mpg highway. Not so good for "sporty" driving as the Fiero's 3.33 but better gas mileage at a lower cruising RPM.
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04:51 PM
sjmaye Member
Posts: 2468 From: Hendersonville, TN USA Registered: Jun 2003
No, That is not accurate. The 1984 thru 1986 had a 3.18 final drive gear ratio. The 1987 and 1988 used a 3.33 final drive gear ratio. It does not matter which engine is used. Aside from the final drive, all the auto's are the same.
You can tell which final drive you have by the RPO codes: F75 Transaxle, final drive, 3.18 ratio GX3 Transaxle, final drive, 3.33 ratio
Same info I got when I was looking at replacing my transmission.
If ya gotta put a tranny in, look into getting a 4-speed. It was used with the 2.5 and 2.8 in various GM front wheel drive models. I think the model is T-440. The fourth gear is overdrive and will boost fuel economy significantly
I'm not sure the 4T60 was ever matched by GM to a 2.5. 2.8's I don't know. I know it was used with 3.1 V6's in the Pontiac 6000. These are the correct left CV axels we use for the Caddie 4t60E swaps. THe Duke was a little weak to handle the ovedrdrive.
[This message has been edited by josef644 (edited 07-28-2009).]
Originally posted by Oreif: The 1984 thru 1986 had a 3.18 final drive gear ratio. The 1987 and 1988 used a 3.33 final drive gear ratio. It does not matter which engine is used. Aside from the final drive, all the auto's are the same.
You can tell which final drive you have by the RPO codes: F75 Transaxle, final drive, 3.18 ratio GX3 Transaxle, final drive, 3.33 ratio
I just checked the top side of the black, driver's side fenderwell of my automatic-equipped, `86 GT, which I've owned since new. Of the 41 codes on it, which I presume to be RPO codes, one of them reads "GX3." None of them reads "F75."
Accordingly, I'm now thoroughly confused: "GX3" presumably designates a 3.33 final drive ratio, but my GT is an `86, and your observation that the GX3 code would designate a 3.33 final drive ratio on my car --- which is an `86 --- is inconsistent with your earlier statement:
quote
Originally posted by Oreif: The 1984 thru 1986 had a 3.18 final drive gear ratio.
I'm not trying to give a fellow N.I.F.E. (Northern Illinois Fiero Enthusiasts) club member grief here, especially since I was hoping that I actually had a 3.18 ratio! If I had a 3.18, then I could improve the car's acceleration slightly by going to a numerically higher, 3.33 ratio. Apparently, however, there would be zero point in my doing that for slightly better acceleration because I presumably already have a 3.33, based on the aforementioned "GX3" RPO code --- despite my Fiero being an `86.
I've gone to "The Ogre's Fiero Cave" that is here on Pennock's Fiero Forum ( http://home.comcast.net/~fierocave ), and it also shows RPO code "GX3" designates a 3.33.
Perhaps there is a typo in the GM-provided manual you cited, Oreif?
For example, on page 3-1 of my original, GM-provided, "Pontiac Fiero 1986 Do-It-Yourself" manual, it reports the same information you cited above, namely, that an `86 automatic would have a 3.18 final drive ratio. However, on page 3-2 of my "Pontiac Fiero 1986 Do-It-Yourself" manual, it also reports, quite erroneously, that the `86 GT has 6-inch-wide wheel rims, rather than the 7-inch wide rims with which it had been equipped originally.
Related to this possible final-drive-ratio-typo by GM, what might be useful here is if owners of other automatic-equipped `86 Fieros inspected the RPO codes on their cars.
Remember, if it has a "GX3" somewhere in the RPO codes' sticker located on the top side of the black, driver's-side fenderwell of your automatic-equipped `86 Fiero, then it presumably has a 3.33 final drive ratio, not the 3.18 indicated in the GM-provided, "Pontiac Fiero 1986 Do-It-Yourself" manual that accompanied each new `86 Fiero.
maybe the switch from 3.18 to 3.33 was a mid year running change so an 86 could have both like the switch to 5 speeds getrags also done at that time
as far as I know the 2.84 and 3.06 were duke only gears that looks to be odd as the duke sure could use the extra go a 3.33 would give and while the V6 was a little slower in my swaped 88 coupe with the 2.84 unit it was not that bad and the lower RPM at speed was nice maybe it was CORP fuel econ thing to meet gov avg for the year
------------------ Question wonder and be wierd are you kind?
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04:55 PM
Oreif Member
Posts: 16460 From: Schaumburg, IL Registered: Jan 2000
I'm not trying to give a fellow N.I.F.E. (Northern Illinois Fiero Enthusiasts) club member grief here, especially since I was hoping that I actually had a 3.18 ratio! If I had a 3.18, then I could improve the car's acceleration slightly by going to a numerically higher, 3.33 ratio. Apparently, however, there would be zero point in my doing that for slightly better acceleration because I presumably already have a 3.33, based on the aforementioned "GX3" RPO code --- despite my Fiero being an `86.
I've gone to "The Ogre's Fiero Cave" that is here on Pennock's Fiero Forum ( http://home.comcast.net/~fierocave ), and it also shows RPO code "GX3" designates a 3.33.
Perhaps there is a typo in the GM-provided manual you cited, Oreif?
For example, on page 3-1 of my original, GM-provided, "Pontiac Fiero 1986 Do-It-Yourself" manual, it reports the same information you cited above, namely, that an `86 automatic would have a 3.18 final drive ratio. However, on page 3-2 of my "Pontiac Fiero 1986 Do-It-Yourself" manual, it also reports, quite erroneously, that the `86 GT has 6-inch-wide wheel rims, rather than the 7-inch wide rims with which it had been equipped originally.
Related to this possible final-drive-ratio-typo by GM, what might be useful here is if owners of other automatic-equipped `86 Fieros inspected the RPO codes on their cars.
Remember, if it has a "GX3" somewhere in the RPO codes' sticker located on the top side of the black, driver's-side fenderwell of your automatic-equipped `86 Fiero, then it presumably has a 3.33 final drive ratio, not the 3.18 indicated in the GM-provided, "Pontiac Fiero 1986 Do-It-Yourself" manual that accompanied each new `86 Fiero.
HMM... I wonder if the 3.33 was a late addition in 1986 like the getrag 5spd?? It maybe the fastback GT's had the 3.33 since they were switching over and the GT was a mid-year start.
As for wanting to change the final drive ratio's, Here is a list of gears you can change with their final ratio's
HMM... I wonder if the 3.33 was a late addition in 1986 like the getrag 5spd?? It maybe the fastback GT's had the 3.33 since they were switching over and the GT was a mid-year start.
You make a good point about the "mid-year" introduction of the `86 GT. That could account for the 3.33 final drive ratio in my `86 GT, as well as the absence of a reference to it in the GM literature for that year.
I suppose a related possibility is that at GM back then, one group, the people developing the Fiero literature, was not working very closely with another group, those who were introducing the `86 GT. I say that for three reasons:
In my original, GM-provided, "Pontiac Fiero 1986 Do-It-Yourself" manual, both pages 3-1 and 3-2 report an `86 automatic has a 3.18 final drive ratio. However, my `86 automatic GT has the 3.33 axle ratio (RPO code "GX3" in the RPO sticker on the driver's side fenderwell), and that is not even referenced in that manual.
Page 3-2 of my "Pontiac Fiero 1986 Do-It-Yourself" manual reports, also erroneously, that the `86 GT has 6-inch-wide wheel rims, rather than the 7-inch wide rims with which it had been equipped from the factory.
Page 3-3 of this `86 manual also erroneously shows a picture of what appears to be an `85 GT because of its rounded front fascia --- coupled with a "notchback" roof --- rather than an `86 GT, all of which had the then-new "fastback" roof.
In short, I suspect either my original-owner "Pontiac Fiero 1986 Do-It-Yourself" manual has at least three mistakes reported in it, or I have an ultra-rare `86 GT --- likely worth trillions.
If the latter is true, that would be nice, as I could use the extra money.
I am confus
