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I've broken this out from the TH125 page because I finally found the math that shows the effect of changing tire sizes on the transmission. I'll add the ratios for the various manual transmissions if/when I find it.
Why do people change the Final Drive? What difference does that make?
There are 2 main reasons to change the final drive of the transmission:
The first is to compensate for a change in tire diameter (Not rim diameter!). Changing tire size alters the performance of the car. Changing the final drive can restore the performance loss that comes with larger tires. While usually percieved as a truck issue, when people change to tires on 17" and larger rims, it will affect the car. Often more then you think.
The second is to increase launch torque or to get better highway mileage. You can have one or the other, not both. Ratios closer to 1:1 get better mileage and higher top speed but low torque. Ratios moving away from 1:1 increase torque at the sacrifice of mileage and top speed.
Note! Doing an AOD (4T60, etc.) conversion? See Also Here.
TH125 & 4T60 Gear Ratios
When most people think about final ratios in TH125c (3T40) and 440-T4 (4T60) transmissions, they simply assume that it means the final drive gears. Problem is things don't work that way in these units. Here are the 10 RPO codes and the three final gear sets that go with them.
| RPO Code | Ratio | Sun Gear |
|---|---|---|
| F16, F17, F62, F75 | 2.84:1 | 38 Teeth |
| FP3, FW2, FW9 | 3.06:1 | 34 Teeth |
| F77, F79, GX3 | 3.33:1 | 30 Teeth |
Please note: The table above is based on information from a GM TSB and the transmission parts catalog from Aceomatic Transmissions. Those really are the only three final gear sets available for the TH125 or TH440-T4.
The RPO list says F62 is 2.39:1. Something isn't right.
Actually the RPO list is correct. Notice that the RPO list says "Transaxle Ratio" not Final Drive or Axle. The confusion is caused by the fact there is something else contributing to total gear reduction besides the final gear set.
Why do some documents say different ratios for the same final gears?
I finally found this explained in yet another GM TSB. Apparently many GM dealer shops had the same question.
Bulletin Number: 89-7-26
Reference Number: 977115
Publish Date: 4/89
125C AND 440-T4 TRANSAXLE FINAL DRIVE
Subject: RATIO
Models: All 1980 - 1989 MODELS WITH 125C (3T40) OR 440T4 (4T60) TRANSMISSIONS
When identifying a 125C/Hydra-matic 3T40 or 440-T4/Hydra-matic 4T60 transaxle for the purpose of ordering parts, there may be conflicting final drive ratios listed in different publications for the same transaxle I.D. code.
Some sources, such as the Chassis Service Manual, may list the actual ratio of the final drive in the transaxle. Other sources, such as the GMSPO Parts catalog may list an "AXLE RATIO" which is figured using ratio of the final drive assembly combined with the drive to driven sprocket ratio of the drive chain.
In English... This means some documents report straight final drive ratio, while others report the final ratio as the actual final gear ratio adjusted by the Chain ratio. (This is why the RPO list doesn't jive with the actual final gears in many transverse transmissions.)
Many people forget that you can have a gear reduction in the chain set in these transmissions. (Many people don't even know there is a chain in these transmissions....) This effectively alters the gearing of the whole transmission just like changing the finals would.
How does this work?
Let's say two TH125c transmissions have 2.84:1 final gears. (Remember that third gear of TH125c and 4T60 is 1:1!)
Transmission 1 has a 1:1 chain or 35 teeth on both chain sprockets. The total gear reduction in this transmission is 2.84:1 in third gear.
Transmission 2 has a .84:1 chain or 38 teeth on the drive sprocket and 32 on the driven sprocket. The total gear reduction in this transmission is 2.39:1 in third gear.
How many sets are there?
For TH125c, there are four chain sets and three final sets totaling 12 possible combinations. Everything for the 4T60 is the same except that 38/32 (0.84:1) chain was not used in that unit.
After originally posting this, partfiero told me there might be a 33/37 chain set. I was able to confirm this chain set actually exists thru the Aceomatic catalog linked above. I think I finally have everything...
Here are all the possible ratios and the RPO codes for ones GM actually built. These RPO codes are additional to the Transaxle Type RPO. This means that on a vehicle's build sticker you'll find MD9 for TH125c (3T40) or ME9 for 440-T4 (4T60) and one of the codes below to tell you how the transmission is geared. (Assuming you still have the original transmission...) The last column reflects a couple spots where GM and MS Excel rounded the number differently.
| Actual Final Gear (X:1) |
Chain Drive/Driven | Chain Ratio (Y:1) | Total Reduction (X*Y:1) | Transaxle Ratio RPO | RPO says (If dif) |
|---|---|---|---|---|---|
| 2.84 | 35/35 | 1 | 2.84 | F17 | |
| 37/33 | 0.89 | 2.53 | F16 | ||
| 38/32 | 0.84 | 2.39 | F62 | ||
| 33/37 | 1.12 | 3.18 | F75 | ||
| 3.06 | 35/35 | 1 | 3.06 | FW2 | |
| 37/33 | 0.89 | 2.72 | FP3 | 2.73 | |
| 38/32 | 0.84 | 2.57 | |||
| 33/37 | 1.12 | 3.43 | FW9 | ||
| 3.33 | 35/35 | 1 | 3.33 | GX3 | |
| 37/33 | 0.89 | 2.96 | F79 | 2.97 | |
| 38/32 | 0.84 | 2.80 | |||
| 33/37 | 1.12 | 3.73 | F77 | ||
| Chain Ratios of less than 1:1 are over driven | |||||
Gear Ratios
Just in case someone needs them...
| RPO | Transmission | 1st | 2nd | 3rd | 4th | 5th | Reverse | Final |
|---|---|---|---|---|---|---|---|---|
| MD9 | THM 125c (3T40) | 2.840 | 1.600 | 1.000 | 2.067 | |||
| ME9 | THM 440-T4 (4T60) | 2.921 | 1.568 | 1.000 | 0.705 | 2.385 | ||
| MT2 | Isuzu (L4) | 3.73 | 2.04 | 1.45 | 1.03 | 0.74 | 3.50 | 3.35 |
| MG2 | Getrag (V6) | 3.50 | 2.05 | 1.38 | 0.94 | 0.72 | 3.41 | 3.61 |
| M17 | Getrag (V6) | 3.31 | 1.95 | 1.24 | 0.81 | 3.42 | 3.65 | |
| M19 | Getrag (L4 perf) | 3.53 | 1.95 | 1.24 | 0.81 | 3.42 | 4.10 | |
| MY8 | Getrag (L4 econo) | 3.53 | 1.95 | 1.24 | 0.73 | 3.42 | 3.32 |
Speed vs. RPM Excel 5.0 workbook.
Thanks to cypres, Illustrated Calculator from MFactory Competition Products. It plots all gears on MPH/KPH and Engine RPM axles.
Fiero Factory Setup
Various people have said Fiero automatic transmissions are all setup as one thing or another. A common statement will go "all the L4 cars got 2.84 finals gears." While technically true, as we've seen final gear ratio is not the only factor. Some of those 2.84 L4s are actually running 3.18:1 transmission ratios thanks to the chain set.
According to GM TSB 88-51 issued December 1988, these are all of the factory TH125c configurations used in Fiero thru 1987. (1988 model info is a collection from 22P. Thanks Raydar. Going by the TC part numbers, I think the info is correct.)
| Year | Trans
Model |
Engine | TC
Code |
Stall
Speed |
Final
Drive |
Chain
Set |
Total
Ratio |
|---|---|---|---|---|---|---|---|
| 84 | PI | L4 | FJ2 | 2060 | 2.84 | 35-35 | 2.84 |
| 85 | PF | L4 | FJ2 | 2060 | 2.84 | 33-37 | 3.18 |
| 85 | CD | V6 | FJ4 | 2060 | 3.06 | 35-35 | 3.06 |
| 86 | PF | L4 | FJ2B | 2060 | 2.84 | 33-37 | 3.18 |
| 86 | CD | V6 | FJ4B | 2060 | 3.06 | 35-35 | 3.06 |
| 87 | PSC | L4 | FJ2B | 2060 | 2.84 | 35-35 | 2.84 |
| 87 | CPC | V6 | FD4B | 2095 | 3.33 | 35-35 | 3.33 |
| 88 | PSC | L4 | FJ2B | 2060 | 2.84 | 35-35 | 2.84 |
| 88 | CPC | V6 | FD4B | 2095 | 3.33 | 35-35 | 3.33 |
Trans Model is on the tag/case under the Neutral Safety Switch.
NOTE: Do not use this table to order parts! You must confirm the model code on the transmission including a year digit that may precede the letters. There are often important internal differences each year even though the models are otherwise the same code. (TC Codes are explained in another article.)
Notice that according to the tables in the TC article... all of the Torque Converters are listed as FWD units and all but one is 2060 Stall. The only real difference is that the V6 units have a heavy duty TC clutch.
I did find the range of final gearing interesting. This helps explain some of the variations in fuel economy people see.
How are final ratios and tires related?
If you increase the tire size, you reduce the effective gear ratio. This is ok if you want highway mileage but not good for drag racing or autocross.
Allot of people want the tire to better fill the wheel well, so lets say you want a larger tire on a 17inch wheel. (I just made up a size for the 17inch. I don't know if you can actually get it.) To find how the new wheel alters performance, we can do a bit of basic math.
We need only three bits of information:
The original tire size, 195/75R14, has a diameter = 25.5 inch (OldTD)
The new tire size, 225/60R17, has a diameter = 27.6 inch (NewTD)
OE Ring & Pinion Ratio 3.06:1 (OERatio)
We're only concerned with the rolling diameter of the tires. Tire profile and rim size aren't important for this. You can get this info from either the tire maker (catalog or web site) or you can use the tire size calculators on the links page.
Note that it is important to know the ratio of the gears that are actually in the transmission. We don't want a published ratio that may have been "adjusted" for whatever reasons by the carmakers. We've also got to keep in mind that the choices of alternate gears are usually very limited. That means we've got to balance the tires to available gear sets.
| OldTD x OERatio | = Effective ratio with new tire |
| NewTD | |
| 25.5 x 3.06 | = 2.83:1 |
| 27.6 |
(Original Math Source: www.offroaders.com)
That means when you change to the 17inch tire, the car performs as if the gears dropped to 2.83:1. You get better top end but will launch more slowly. It's the same effect as changing the TH125 to 2.84 gears with the stock tire.
To get back the same performance we need a new set of gears, to find them we run another calculation...
| NewTD x OE Ratio | = New gears required |
| OldTD | |
| 27.6 x 3.06 | = 3.31:1 |
| 25.5 |
A 3.31:1 gear set would return the car to the same performance as original. The closest TH125 alternative is 3.33:1 so that works out well for the new 17" tire. You'll end up with performance as if the car was geared down very slightly. That means slightly faster launches without totally squashing fuel economy.
Note that since Fiero VSS reads Axle rotations, Speedometer output doesn't change relative to final gears. Speedo recalibration is yet another calculator. Nearly all the transverse transmissions have the speedo reading axle rotation vs. traditional RWD where the speedo reads drive shaft rotations at the transmission.
For most people with an automatic, the best way to change final drives will be to change the whole transmission. This is because the governor in TH125 and several other GM transverse transmissions reads axle rotation. Changing finals without correcting the governor function could mess up shift timing. There may be other factors involved as well. This puts changing finals out of the reach of most DIY folks.
Even for the stock manual transmissions, it's probably better to change the whole transmission than try to change the finals. Manual transmissions may seem easier to work with but they really aren't. You can easily cause all sorts of problems unless you handle them very carefully. The problems may not show up right away.
Depending on how the ECM is programmed, changing finals could also set error codes. I'm not aware of this being a problem in Stock Fiero but some of the engine conversions could have a fit if the finals are too far from what the ECM/PCM expects.
When used with one of the Java based tire size calculators (In the links section) you can work all the numbers fairly quickly for a whole range of tire sizes. The math above will tell you how bad it will hurt performance. The tire calculator will tell you how far off a new tire will put the speedometer. The tire calculators can also be used to find various tire sizes that match OE diameter or will work with different gear sets.
Any tire with 25.5inch diameter, such as 225/55R16 (25.6inch diameter), can replace the 195/75R14 without significantly messing up performance or the speedometer no matter what the rim size is. That 16inch tire is only one tenth of an inch larger. You get a short sidewall for better handling but only mess up the speedometer by 0.9% vs. a 3% error changing to a 195/70R14.
Likewise changing from 3.06 to 3.33 gears would work for any tire of similar diameter to our sample 17inch one.
On the other hand, if you've already got 3.33:1 gears in a TH125 then you'll have no choice but eat a performance change if you install larger tires unless you can find aftermarket gears with a higher ratio.