Gearbox / speedometer problem

The automatic transmission in my 1990 Mk1 Clipper died, so I had a used
replacement put in.  It transmits the power just fine, but the
speedometer/odometer now registers about 1/3 too little.  I've tried
replacing the speedometer, the speedometer cable, and the white plastic
cable drive gear (which inserts in the transmission housing at the bottom
of the cable) with no improvement.

So, I am left with assuming I have either the wrong transmission, or a dud
transmission.

I've tried to check the former with VAG, but it doesn't much care.  There
may have been two speedometer drive ratios, but again there may not have been.  I found a speedometer from a Jetta with a different ratio printed on its face, but it didn't register accurately either (although it had a
different amount of error from that of my original speedometer)

There were a lot of different transmission models in these cars, but the
wrecker who sold me the replacement (a trusted friend and VW specialist)
said they will almost all work OK.  He is willing to exchange the one he
sold me for another replacement, free, but of course I will have the risk
of whether the new one will work properly and the significant cost of the
installation, so that is a last resort.

What could be causing it that I haven't thought of?  

Not having an accurate speedometer is bad in a land of speed cameras, and not having an accurate odometer is bad for proving business distance to the taxman.

Thanks for any help.

Geo.

Hmmm,

Not heard of that happening before. I wouldn't have thought there was much variation in the auto boxes for the cabriolets. Strange.

Anyone?

Cheers,

Drew.

Have heard of asimilar problem of this befor. ther are two diffrent speedo drives white and red. My car has a red drive but it is a manual, maybe worth trying a red drive befor taking the box out.

Cheers
Craig