If your speedo is electrical and programable find out how many pulses per second that your sensor puts out. Certain programmable guages can only read a certain range. The other option is to see if your VDO gauges offers a pulse sender in place of yours which can also correct the problem. If your problem persits give these guys a call there website is Speedometersolutions.com
They can do anything, they even took my old mechical driven speedo cable and put a pulse generator on it so I could change my system from mechanical to electrical. It is pretty neat stuff and all for $90 bucks not bad. I hope this helps
