While I was driving my car suddenly the left control arm suddenly broke off (the car has had a curb accident on that side before) and the cv-joint de-attached from the transmission and the car was not moving anymore while in gear. But something I noticed is the vehicle speedometer was relative to the RPM; as the car was in gear and increasing the RPM the speedometer was functioning as if the vehicle was in motion. The car in question is a regular FWD with Front Engine Layout.

So does the ECU just use the rpm, gear ratio of the current gear and final drive ratio to calculate vehicle speed? what if the engine wasn't so healthy and was not producing the torque it used to say 10 years ago at the same rpm, is the speedometer now inaccurate because the motor losing its power? Is it calculated differently for different engine/drive setups?


3 Answers 3


If the ECU would calculate speed from RPM, how would it calculate speed when the car is in neutral or during shifting?

The speed usually is derived from the RPM of the output shaft of the transmission (to the differential). This way, one gets an average of the speed of the left and right wheels.

The key to your observation is the differential. It distributes the power to the left and right wheel while allowing both wheels to turn independently. When one wheel is on the ground and the other is lifted, only the lifted wheel will turn when the motor is running and the transmission in gear, while the other stands still. The speedometer will indicate a speed because the shaft to the differential is spinning, while the wheel turns with twice this speed. (Remember: Displayed speed is average of left and right wheel)

Exactly the same happens if one drive shaft to a wheel is broken.


Somewhere there is a signal that measures rotating speed on the driveline, downstream of the torque converter. It is typically somewhere inside the transmission housing. Because of how a torque converter operates, engine RPM would not be a totally great indicator for vehicle speed.

Typically auto manufacturers use a hall effect sensor to determine spinning shaft speed. Those sensors are pretty cheap. Having one sensor for engine RPM, and another for vehicle speed would give great feedback to the engine control computer on the functioning of both the engine AND the transmission, including torque converter. Note: it would certainly be possible to use the ABS (Anti Skid Brake Control System) to determine vehicle speed. An ABS vehicle has a notched ring at each wheel, with hall effect sensor and is used to determine the speed of that individual wheel. By looking at the averages from all four wheels, a computer can determine vehicle speed.

I've seen vehicles with hall effect speed sensors on Camshaft, Crankshaft, transmission output shaft and ABS wheel sensors... go figure.


The speedometer is generally connected somewhere after the gearbox eg the driveshaft. So in normal circumstances this is directly proportional to the angular velocity of the wheels, depending on the differential ratio and assuming no wheel slip. This needs to be somewhere between the clutch and the differential otherwise it would jump around whenever the clutch was disengaged. Placing it after the gearbox also removes the need to sense and compensate for individual gear ratios which would be more difficult in a mechanical system.

The relationship between torque and rpm is irreverent here as the speedometer is calibrated from the rotational speed of the wheels and their diameter.

So if a transmission component fails downstream of where the speedometer is connected it can't tell that the wheels are no longer turning, essentially the same situation as if the drive wheels are jacked off the ground or spinning.

typically regulations require that the indicated speed is never less than the actual speed but allow a margin of error above actual speed (in some cases this can be quite large).

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