dci speedo problems

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So my brother in law has bought a 2003 dci 65, and although it drives ok the speedo reads lower than it should. 30 is reading around 22 and 60 is only showing about 43 on the instrument cluster.

I'm not overly up to speed on the whole ABS driven speedo system but hopefully someone can point me in the right direction please

I've run a diagnostic test with my machine and apart from a whole host of old airbag faults (wiring under seats) there wasn't anything amiss, however it wouldn't even recognise the ABS system at all, I've since connected to the Mrs dci and works fine so I'm fairly confident it's not diagnostic related

So far I've checked the front reluctor rings, they are the 44 tooth version (I had wondered if someone had replaced them), the nearside front sensor does appear a little scored bit would this enough be causing speedo error? Or am I going in totally the wrong direction with this? It's my understanding that it's this sensor that is used to determine road speed

Hopefully someone can point me in the right direction, otherwise I'll probably end up being stuck with it and using it for parts to keep the Mrs one going

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