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How is speed detected?


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Today the roads were pretty bad and there was quite a bit of wheelspin (yes the wheelspin was my fault).

What caught my attention was how little the speedo jumped even at WOT. I realize the traction control was on but I would've thought that the displayed speed would increase by at least a few MPH at full throttle. Later on, travelling with my friend in his Dodge and seeing the speedo peak at 200km/h while we weren't even moving emphasized the difference.

I thought maybe the sensor for the instrument panel speedo might be linked to the rear wheels but a quick E-brake proved that wasn't the case.

So, where is the sensor, and how is it almost unaffected by wheelspin?


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It's my understanding the wheel sensors for the antilock brakes work as input for the traction control.

You have vehicle speed input,(speedometer), and if one wheel (spinning) is detected (by that wheel sensor), the computer knows that wheel is turning faster than it should. Then the brake is applied to that wheel to stop the spinning.

If the brake is unable to control the spinning, then the computer begins to shut down the fuel injectors to limit power from the engine so it won't over power the brake that is trying to control the spin.

Hope that makes some sense.


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