speedo transducer sensor, what does it do?

[SVT8]

iv got to replace my speedo transducer, but what is its function?. It makes the speedo needle play up when idling on occasion, but it also causes the engine to missfire when its doing this. What does this sensor actually control?

Also what other problems can this sensor cause?

[Jastel]

It drives the speedo.
If the speedo is reading wrong the ECU gets unhappy and car runs bad.
It need accurate speed for revs and gears etc.