Transmission gear ratios
Transmission gear ratios lists the commonly referenced factory transmission gear ratios used in the Pontiac Fiero.
The Pontiac Fiero used several different manual and automatic transaxles from 1984 through 1988. Gear ratios varied by transmission type, RPO code, engine, and model year.
Because many Fieros have been modified, repaired, or transmission-swapped, the actual installed transaxle should be identified directly when exact gearing matters.
Overview
A gear ratio describes the relationship between engine speed and transmission output speed in a particular gear.
A numerically higher gear ratio provides more mechanical advantage but produces higher engine rpm for a given road speed. A numerically lower gear ratio provides less multiplication and can reduce cruising rpm.
The final-drive ratio also affects the overall gearing. In a Fiero transaxle, the transmission and differential are combined in one unit.
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Factory transmission RPO codes
| RPO code | Transmission | Common name | Factory Fiero application |
|---|---|---|---|
| MD9 | 3-speed automatic | TH125C automatic | 1984-1988 automatic-equipped Fieros |
| MY8 | 4-speed manual | Muncie 4-speed | 1984 four-cylinder economy |
| M19 | 4-speed manual | Muncie 4-speed | 1984 four-cylinder performance |
| M17 | 4-speed manual | Muncie 4-speed | 1985-1986 V6 |
| MT2 | 5-speed manual | Isuzu 5-speed | 1985-1988 four-cylinder |
| MG2 | 5-speed manual | Getrag 5-speed | Later 1986-1988 V6 |
Manual transaxle ratios
Factory manual transaxle ratios varied by RPO code, engine, and model year.
| Years / application | RPO | Transmission | 1st | 2nd | 3rd | 4th | 5th | Final drive |
|---|---|---|---|---|---|---|---|---|
| 1984 four-cylinder economy | MY8 | Muncie 4-speed | See note | 1.95 | 1.24 | 0.73 | — | 3.32 |
| 1984 four-cylinder performance | M19 | Muncie 4-speed | 3.53 | 1.95 | 1.24 | 0.81 | — | 4.10 |
| 1985-1988 four-cylinder | MT2 | Isuzu 5-speed | 3.73 | 2.04 | 1.45 | 1.03 | 0.74 | 3.35 |
| 1985-1986 V6 | M17 | Muncie 4-speed | 3.31 | 1.95 | 1.24 | 0.81 | — | 3.65 |
| Later 1986-1988 V6 | MG2 | Getrag 5-speed | 3.50 | 2.05 | 1.38 | 0.94 | 0.72 | 3.61 |
The 1984 MY8 economy 4-speed first-gear ratio should be verified from factory documentation before being listed definitively. Some enthusiast sources list it differently than other commonly repeated Fiero ratio tables.
Automatic transaxle ratios
The Fiero factory automatic transmission was the TH125C automatic, associated with RPO code MD9.
| Gear | Ratio |
|---|---|
| 1st | 2.84 |
| 2nd | 1.60 |
| 3rd | 1.00 |
| Reverse | Commonly listed as approximately 2.07 |
Automatic final-drive ratios
Factory TH125C final-drive ratios varied by model year and transmission code.
| Model year | Final drive | Code | RPO |
|---|---|---|---|
| 1984 | 3.18 | PF | MD9 |
| 1985 | 3.06 | CD | MD9 |
| 1985 | 3.18 | PF | MD9 |
| 1986 | 3.06 | 6CD | MD9 |
| 1986 | 3.18 | 6PF | MD9 |
| 1987 | 3.33 | 7CPC | MD9 |
| 1987 | 2.84 | 7PSC | MD9 |
| 1988 | 3.33 | 8CPC | MD9 |
| 1988 | 2.84 | 8PSC | MD9 |
Automatic transmission codes and final-drive ratios should be verified when exact identification matters, especially if the transmission may have been replaced.
Effect of gear ratios
Gear ratios affect acceleration, cruising rpm, fuel economy, and drivability.
A numerically higher first gear or final-drive ratio can improve low-speed acceleration, but it can also increase engine rpm at a given road speed.
A numerically lower final-drive ratio can reduce cruising rpm, but may reduce mechanical advantage during acceleration.
The overall drive ratio in any gear is affected by both the selected gear ratio and the final-drive ratio.
Overdrive
An overdrive gear has a ratio below 1.00:1.
Fiero transmissions with an overdrive gear include:
The TH125 automatic does not have overdrive.
The Muncie 4-speed does not have a fifth gear, but some versions have a fourth-gear ratio below 1.00:1.
Identification cautions
Gear-ratio information should not be based only on the car's model year or trim level.
Many Fieros have been repaired, modified, or transmission-swapped. A car may no longer have the transmission it originally left the factory with.
When exact gearing matters, verify:
- RPO code
- Transmission code
- Transmission case features
- Shift pattern
- Model year and engine application
- Service parts identification label
- Physical inspection of the installed transmission
- Previous-owner documentation
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Relationship to engine swaps
Transmission gearing is an important consideration in engine swaps.
A transmission that works well with a stock 2.5L four-cylinder or 2.8L V6 may not be ideal for a different engine.
Engine-swap transmission choice can depend on:
- Engine torque
- Intended use
- Highway rpm
- Acceleration goals
- Transmission strength
- Clutch compatibility
- Mounts and brackets
- Shift cables
- Axles
- Parts availability
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Notes
Published Fiero gear-ratio tables are sometimes repeated from enthusiast sources and may contain small differences, especially for early 1984 four-speed applications.
For restoration, concours, racing, or detailed technical work, verify ratios against factory service information, transmission codes, or direct inspection.