Muncie M21 Gear Ratios – What You Need To Know

We have introduced you to the gear ratio of the 700r4. Today, here comes another transmission: Muncie M21.

If you are a car lover, you must have heard of the Muncie four-speed M21, one of the most popular transmissions on the market.

Still, laypeople might not have insights into this transmission. Are you one of them wondering what the Muncie M21 gear ratios are?

Let’s find out with this article!

What Is Gear Ratio?

Muncie M21 Gear Ratios

The gear ratio is the proportion of the number of rotations the output shaft produces to one rotation of the input shaft.

This ratio describes the difference in rotational speed between the transmission output and the car engine.

Gear ratios may also be used to explain the relationship between tires, differentials, rear ends, and other geared features.

The usual buyer won’t have any special reason to be concerned with the precise drive gear ratio of the car they’re considering.

The manufacturers have meticulously calibrated their vehicles to have the best ratios for each gear under normal driving circumstances.

What Is Muncie M21 Gear Ratios?

The Muncie M21 was originally intended to be paired with 4.56:1 or 4.11:1 GM 4 Series differentials.

Shift points are kept close together in this M21 close-ratio gearbox, which maintains cruise RPMs (revolutions per minute) high.

You may shift gears quickly and maintain the engine’s power band using this close-ratio box, improving performance in drag racing and street racing.

Other Muncie M21 Transmission Characteristics

Below are other characteristics that will help you understand more about the Muncie M21 4 speed transmission.


The Muncie M21 was designed for high-performance automobiles.

This close-ratio box allows quick auto gear changes while maintaining the engine’s power band, enhancing street and drag racing performance.

Input Shaft

M21 transmission features input shafts with 26 splines. Thanks to a strong input shaft, it can provide more torque rating.

The input shaft of the Muncie M21 transmission has a ring groove and a counter gear.

The grooves were carved into the input shaft to make it simple for the transmission assembly line worker to determine which input shaft goes where in a specific unit.

Output Shaft

The driving wheels and M21 transmission are connected by an output shaft, which transfers transmission power to the wheels.

M21 has a high gear ratio, meaning the output shaft can also handle more power compared to other transmissions, such as the M20 Muncie.

Oil Pump

In terms of oil pump level, the M21 transmission will require around three pints or around one and a half gallons.

The transmissions’ oil-fill ports are on the side of the device, making it challenging to change and fill the oil.

To complete this task, you will need a hose or a plastic oil-change pump from an automobile parts shop.


When putting it on a scale, you will find that the M21 transmission’s dry weight is around 155 pounds. The standard transmission weighs anywhere from 100 to 400 pounds.

Thus, as you can see, the weight of the Muncie M21 can be considered quite average compared to other products on the market.


The price for a Muncie M21 can be quite high, from 400 dollars up to a couple of thousand.

Although the cost might be a drawback, it is still understandable, given the quality and strength the transmission provides.

How To Tell M20 From M21

When selecting the best gearbox for your vehicle, you may be curious about the distinctions between the M21 and M20 Muncie 4 speed gearboxes.

Here are a few significant differences between the two.

Gear Ratios

The gear ratios are the main distinction between the Muncie M20 and Muncie M21.

The former is a wide-ratio gearbox with a gear set that has a 1st gear ratio of 2.52:1 and a 4th gear ratio of 1.91:1, which is more appropriate for street automobiles.

Meanwhile, the Muncie M21 close-ratio transmission has a gear set with a 1st gear ratio of 3.36:1 and a 4th gear ratio of 0.68:1, making it ideal for high-performance automobiles.


Since the M21’s gear set is more robust than the M20’s, there is a weight difference between the two.

The dry weight of the M20 transmission is roughly 140 pounds, and the dry weight of the M21 transmission is roughly 155 pounds.

The M21 transmission is recommended for high-performance applications because of its stronger gear set’s increased durability and endurance.

Input Shaft

Both the M21 and M20 transmissions have input shafts with 26 splines. The M21, however, comes with a stronger input shaft, which allows for a higher torque capacity.

The gear ratio for the M21 will differ from the gear ratio for the M20, often being numerically greater.

Due to this, fuel efficiency is marginally improved, and engine speeds are marginally reduced when traveling at highway speeds.

Output Shaft

Another factor that differs between Muncie M21 and M20 is the gears and output shafts.

The M21 will have a faster peak speed than the M20 since the M21 comes with a higher center gear ratio.

Compared to the M20, the M21’s gear set is stronger and can withstand more force and abuse.

Pressure Plates And Clutch Discs

A Muncie M21 or M20 transmission’s clutch discs are a frequently misunderstood yet crucial component.

The clutch can be smoothly engaged and disengaged thanks to clutch discs, which also convey torque from your car engine to your transmission.

Additionally, it assists in protecting the transmission and engine from severe shock loads.


Compared to the M20 transmission, the M21 is more expensive. Unlike the M20, which has a wide ratio, the M21 has a close ratio.

In order to handle more power, the M21 is also composed of stronger materials. The M21 is superior for performance vehicles, while the M20 is superior for everyday transportation.

Frequently Asked Questions

How Much Horsepower Can The Muncie M21 Handle?

The OEM M21 Muncie 4 speed transmission is exceptionally robust and resilient and can manage a high horsepower level.

It is frequently utilized in high-performance applications with 400 horsepower.

It is also a well-liked option for off-road applications because of its strength and durability, which are highly valued.

Which Is Better: Close Ratio Or Wide Ratio Transmission?

It depends on your personal preference. In a wide-ratio gearbox, the gear ratios between the first and second gear, the third and 1:1 fourth change more abruptly.

Despite being more progressive, a close gear ratio ends at 1:1 in the fourth gear.

The ideal transmission ratio is determined by your code rear axle gear ratio and the amount of track time you will get.

Choose the close-ratio transmission if your gear ratio is 3.73:1 or below and is suitable for the street.

Meanwhile, go for the wide ratio when your gear is steeper and you don’t care much about driving ability.

What Year Did The Muncie M21 Come Out?

The Muncie M21 was officially introduced in 1963.

In actuality, 1963 stands alone as a different year since alterations to this box began almost immediately in 1964, starting with a front bearing cage retainer made of aluminum in 1963 but changed to cast iron in 1964.

How To Calculate Gear Ratio?

To calculate the cluster gear ratio, you can use the following formula: Gear ratio = T2/T1. T1 denotes the first gear’s tooth count, and T2 is the second gear teeth count.

The Bottom Line

How To Tell M20 From M21

So there you have everything you need to know about Muncie M21 gear ratios and the transmission’s other characteristics.

If you desire a more advanced transmission, the Muncie M21 is preferable because it features an overdrive and can hold up better over time.

Also, we have covered many problems, such as manual transmission going into gear but won’t move, on our site.

In case you encounter these issues, you can check them out and get the answers you need.

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