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Sep 12, 2023
Tinton Falls, NJ
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Gears will increase acceleration but decrease top speed and fuel economy. Final drive gear ratio is associated with the differential in your car. The internal combustion engine does not make enough torque to move a car (at any kind of speed) so gearing is used to multiply torque output in transmissions and final drives. Check the sticker on the driver side door for rear axle codes. Here is the list of codes and what they mean:

Look under the "AX" on the door jam sticker for your axle code; these gears were found in MN12s and FN10s from the factory:

ConventionalTraction-LokFinal Drive Ratio
5 (W5)E (XE)3.27

These gears were not factory-equipped in MN12s or FN10s, but may be found in other vehicles should you wish to install them in your car:

ConventionalTraction-LokFinal Drive Ratio

All 3.8L V6 N/A cars were equipped with 3.27s. The 89-92 model years got a limited slip differential (Traction-Lok/TL) and the 8.8" gearset if they came with ABS, and an open 7.5 if they had regular brakes. All SCs and V8 cars got the 8.8" gearset. The manual SCs all got 2.73 TL. The 89-93 auto SCs got 3.27 TL, and the 94/95 auto SCs got a 3.31 open, with TL as an option. 91-93 5.0 cars could have 2.73, 3.08, or 3.27 - either open or TL. 94/95 4.6 cars got 3.08 or 3.27, open or TL. All 96/97 cars got 3.27s, with TL being an option. The base Mark VIIIs got a 3.07 open; the LSC models were equipped with a 3.27 open.

As a guide in changing rear end ratios, use the following formula.

[new gear ratio/old gear ratio] = RPM factor increase (or decrease) for a given speed.

For example, should you have 2.73:1 gears now and are turning 1,500 RPM at 65 MPH in high gear now; and you change to 4.10:1 gears, then you will be turning 2,250 RPM at 65 MPH with the new gears for a factor of 1.5x for the change.

If you go from a 3.27 to, say, 3.73, you will increase your RPM at any given speed by a factor of 1.1406 x or an increase from 2,000 rpm in OD at 70 MPH to about 2300 rpm at the same speed. From 3.27 to 4.10 is an increase of 1.254X or about 2,500 RPM for the same speed.

The PCM is capable of adjusting shift points automatically within a 20% margin of the factory speedo gear. If your RPM factor increase (or decrease) exceeds 20%, you will need a tune to properly set the shift points.

Which speedo gear to put in?

Determining which speedo gear you need is not as simple as looking at a chart with your final drive ratio and number of teeth on your output shaft - tire size must be factored in as well. The formula to determine the number of teeth your speedo gear needs is:

Tire rotations per mile x number of teeth on transmission output shaft x final drive ratio / 1000

All FN10s and MN12s with the 4R70W have a 7 tooth drive gear for the speedometer on the output shaft of the transmission. The older MN12s with the AOD (or 4R70Ws from other platforms) have 8 teeth on this gear.

If the result of your calculation falls just below the midpoint between two gears (e.g. 19.45), I recommend you round up instead of down. This is because as your rear tires decrease in diameter as they wear, the result of your calculation increases. Thus as your tires wear, your speedo gets more accurate instead of less accurate.

A tire size and speedo gear calculator can be found here.

TransmissionTeethColorPart Number

The 21 tooth gear is the finest gear that can be safely used in the 4R70W transmission, and in most applications for our cars, will support a final drive ratio of up to 3.73:1. An aftermarket 23 tooth gear is made that technically would work with 4.10 gears, but it wears out quickly and is not recommended.
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