Overpaid Train Driver

Hi Guys, my Speedo is fluctuating, it always goes down and back up again but sometimes sticks at zero. Can anyone point me in the right direction please? 

Oh, and where does the Speedo get its feed from?

TIA

John

Good evening guys, just joined the forums.

As you can see from the title, I'm the proud owner of a pair of Audi's finest. A 2004 S4 convertible and a 2001 S6, both have had long spells out of use and I'm slowly bringing them back to rude health.

Look forward to picking your brains (which will probably be quite often) and chipping in my odd 2p worth.

John