2002 bmw 320d speedo confused

[New2bmw]

So I've bought a 2002 320d with speedo, mpg gauge, abs not working.

Now here is why I'm confused because every where I turn websites and forums say abs speed wheel sensor. I changed one on advise I got told it was front left cos that's the one the speedo goes from. Then my mechanic mate phoned someone up and they said this model goes from the gear box. So again I come home look over the net, I can't find any parts for speedo drive cable. Still everything points at wheel sensors. After changing the one abs wheel sensor the abs tract lights go of when I start the car but after a few seconds they come on. The speedo and mpg gauge flicker a little now and then. 

So my question is should I be looking at wheel sensors or speedo drive cable?

 

Thank you

Jon