How do speedometers work without a cable?

Dear Car Talk

Dear Car Talk | Sep 01, 1996

Dear Tom and Ray:

I have just recently purchased a 1993 Dodge Caravan. I was mystified when I
heard that the speedometer doesn't have a cable anymore. Could you explain
how the speedometer works without a cable?-- Steve

RAY: Will miracles never cease! Next thing you know, they'll have spray-on
hair in a can.

TOM: The speedometer cable used to run from the output shaft of the
transmission right up to the back of the speedometer on the dashboard. Then
the speedometer translated the spinning of that cable into miles per hour.

RAY: But then the Vehicle Speed Sensor (VSS) came into being. The VSS is an
electromagnetic pickup at the output end of the transmission which reads
the number of times the driveshaft turns (or the axle if it's a front-drive
car) and sends that information to the engine's computer. The computer then
interprets that data and figures out how fast the car is going.

TOM: And even before it occurred to anyone to use this data for the
speedometer, that VSS information was used to control other devices, like
the cruise control and the electronically controlled automatic

RAY: From there, it was hardly a giant leap for mankind to create a
speedometer that used this same VSS information that the computer was
already receiving. So now, on many cars, the VSS reads the vehicle speed
and sends a signal to the computer, which sends a signal to the
speedometer, which sends a visual signal to you, which explains why there
are sirens and red lights in your rear-view mirror.

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