Does Transmission Control RPM? Answered!  

So you know that your tachometer shows the rpm values of your vehicle and when you shift gears you notice the rpm value changing. 

Now you are wondering, Does Transmission Control RPM?

Yes, transmission controls the RPM of your vehicle whether it’s a manual or automatic type. The difference is, in manual, you initiate the shifting process whereas in automatic it’s electronically controlled. 

To get an in-depth understanding of how the transmission system controls and regulates engine RPM, continue reading. All types of transmissions have been covered in this article. 

Does Transmission Control RPM?

Yes, transmission controls the RPM of a vehicle. Vehicle rpm is the measure of transmission outlet shaft rpm. There is a transmission case that contain all the gears needed to speed up or slow down vehicle as needed. Tranmission with the help of sensors measure the rpm and shift gear to control vehicle speed or tire rpm.

In modern vehicles, both manual and automatic transmissions are equipped with sensors that monitor the RPMs of the input and output shafts. In an automatic transmission, the transmission control module (TCM) and powertrain control module (PCM) receives data from these sensors and use it to determine when to shift gears.

A car’s engine RPM also changes as you drive. Engine RPM is the measure of crankshafts rotation in one minute. So when a driver hits the accelerator the Engine RPM goes up, when he released this pedal the RPM goes down. 

The TCM and PCM communicate with the engine control unit (ECU), which ultimately controls the engine RPMs. In a manual transmission, the RPMs of the input and output shafts of the transmission case are monitored by sensors and sent to the ECU, which then adjusts the fuel and air mixture to control the engine RPMs. 

The driver must manually shift gears in a manual transmission, but the ECU still plays a role in controlling the RPMs to ensure optimal performance and efficiency.

The transmission system with the transmission control module keeps the RPM in check. It keeps the RPM in the right range for the speed and driving condition you are in. This helps the car reduce fuel consumption and increase efficiency. 

How Does Transmission Control RPM? 

The way a transmission system works is, it takes in the engine rpm through the input shaft, and depending on the applied gear, another rpm comes out from the output shaft. This rpm is then transferred to the vehicle wheels. 

Engine RPM is a measure of how fast the engine is running. The rpm that comes out of the transmission is a measure of speed that will be transferred to the wheels. Now we will look at different types of transmission to see how they control RPM.  

How Manual Transmission Controls RPM

In a manual transmission, the RPMs of the engine are monitored by sensors, and the ECU uses this information to adjust the fuel and air mixture to control the engine RPMs. The driver must manually shift gears in a manual transmission since the ECU does not directly control the shifting of gears here.

However, the ECU still plays a role in optimizing the performance and efficiency of the vehicle by adjusting the engine RPMs based on input from various sensors, such as the throttle position sensor and the oxygen sensor. This helps to ensure that the engine is running efficiently and that the vehicle is performing at its best.

The standard type of transmission is a six-speed transmission which means it has six forward gears. A six-speed manual transmission will have six speeds that are applied at appropriate driving conditions. 

The normal rpm rate on a highway is 1500 to 2000 rpm and the idle rate is 600 to 1000 RPM. In manual cars, the driver needs to keep an eye on the rpm and change gears accordingly.  

Here is how the transmission controls your rpm using the driver’s input in a manual car; 

First the driver presses the clutch pedal and turns on the ignition to start the vehicle and shift into first gear. 

The gears engaged in first gear with the specific gear ratio will then control the incoming RPM from the engine to a different lower RPM value. Then when the driver reaches 3000 rpm, he shifts to 2nd gear.

Just like the first gear, the rpm gets controlled again by the specific gears engaged.  After reaching the 2nd gear, the driver can continue to accelerate at the 2nd gear smoothly. They can go up to 6000 to 7000 rpm depending on the vehicle. 

Now they can continue shifting to higher gears while the vehicle speed is increasing. In higher gears, the gear ratio will be low but the output speed will be higher than the original engine RPM. 

Similarly, the driver can go down the gear shifting while reducing the vehicle speed. 

Now one important question is, What RPM should transmission shift?

In a manual transmission, you are supposed to shift at the 2500-3000 rpm range. 

Here is an example of gear ratios available in a manual six-speed transmission. This is ZF six-speed transmission that was used in the Corvette C4, among many other cars. 

The redline rpm for this engine was around 6000 rpm. 

Here is a formula to calculate the output RPM of the transmission;

Let’s say the engine RPM is 3000 RPM, then following this formula the output RPM is given in the table below. The gear ratio of the transmission is a number that represents the ratio of driving gears divided by driven gears.

Different gear ratios represent the different sets of gears engaged during a gear shifting. So, in the first gear, the engaged gears will adjust the input speed to 1119 rpm when it goes to the output shaft.

Gear	Gear Ratio	RPM at Output Shaft
1st	2.68	1119
2nd	1.80	1667
3rd	1.31	2290
4th	1.00	3000
5th	0.75	4000
6th	0.50	6000

The input speed 3000 RPM goes through the transmission and the RPM at transmission output shaft comes out according to the applied gear ratio. Higher gears will increase the RPM as can be seen from the table. 

Summary

A manual transmission takes in driver input via a stick or lever and engine rpm gets controlled. 

How An Automatic Transmission Controls RPM

In an automatic transmission gear shifting happens without the driver pulling a lever. It uses gears, fluid, sensors, etc. to change gears instead. So, through this automatic gear shifting RPM gets controlled. 

The TCM controls the shifting of gears in an automatic transmission by using solenoids to regulate the flow of transmission fluid, which in turn controls the movement of the various gears in the transmission. The TCM sends signals to the solenoids to engage or disengage certain gears based on the RPMs and other sensor data.

The TCM can also control the engine power coming to transmission in an automatic transmission by adjusting the torque converter lock-up clutch, which helps to transmit engine power to the transmission. By adjusting the lock-up clutch, the TCM controls the engine power. 

Hydraulic fluid pressure is used to change gears based on the input from the throttle position sensor and vehicle speed sensor. The TCM takes in all the data and shifts gears at the appropriate time in automatic transmission. 

Here is a list of modules that are responsible for sending data to the engine control unit that controls the transmission.

1. Mass Airflow Sensor: Measures the amount of air intake into the engine
2. Throttle Position Sensor: Measures how open the throttle valve is 
3. Manifold Absolute Pressure Sensor: Sends instant manifold pressure data to ECU 
4. Intake Air Temperature Sensor: Measures the temperature of intake air 
5. Differential Speed Sensor: Measures differential speed 
6. Overdrive Switch: Turns automatic overdrive system On/Off
7. Vehicle Speed Sensor: Measures wheel speed
8. Air Conditioner Sensor: Regulates evaporator temperature
9. Brake Switch: Detects when the brake is applied 
10. Coolant Temperature Sensor: Detect engine operating temperature
11. Turbine Shaft Sensor: Measures RPM of engine 
12. Transmission Fluid Temperature Sensor: Monitors transmission fluid temperature

All these sensors work together and sends data to the transmission control module to shift gears at a suitable time. Now, we will look at an example of how an automatic transmission controls rpm through gear shifting. 

Let’s say the redline rpm of this vehicle is 6000 RPM. Redline RPM is the rpm limit of your engine for safe engine running and you can rev your engine up to this point safely. Here are the available gear ratios and output RPM when the engine rpm value is 4000. 

Gear	Gear Ratio	RPM at Output Shaft
1st	3.10	1290
2nd	1.81	2209
3rd	1.41	2836
4th	1.00	4000
5th	0.71	5633
6th	0.62	6451

Now you might be asking, at what rpm gear shifting happens in an automatic car? 

In an automatic transmission, the gear shift happens in the 2000 to 3000 rpm range. 

If you are wondering who has the best transmission, well we have answers. 

Summary

In an automatic transmission, gear shifting happens due to inputs from sensors, and rpm is controlled electronically. 

How Automated Manual (DCT) Transmission Controls RPM

An automated manual or dual-clutch transmission works like an automatic transmission when it comes to control over gear shifting. This means the driver does not need to grab clutches to shift gears. 

The dual-clutch transmission system employs two clutches to control gear shifting; one for odd gears and another for even-numbered gears. This is found in many cars including the BMW Z4 which has a redline rpm of around 7500.

If the engine is running at 3500 rpm, here is how the transmission affects that in different gears. 

Gear	Gear Ratio	RPM at Output Shaft
1st	4.78	732
2nd	2.933	1193
3rd	2.153	1625
4th	1.678	2085
5th	1.39	2517
6th	1.203	2090
7th	1	3500

All seven speeds coming out of the DCT transmission is electronically controlled. The DCT provides more fuel efficiency and less carbon emission since the gear ratios are closely clustered together. 

Generally, this is how transmission controls the RPM of a vehicle. However, a faulty transmission won’t be able to perform optimally. Here is what happens if you have a sluggish transmission. 

FAQs

Bottom Line 

We expect this article has sufficiently answered your question about the control of rpm using the transmission system. Even though we only calculated six-speed transmissions here, 8-speed and 10-speed work in a similar fashion.