I may be completely wrong here but if you have an electronic speedometer they work off a sender unit and have nothing to do with the voltage  regulator , the voltage regulator usually affects temperature gauges etc. The speedo if like mine is fed from 12-14 Volts and a sender picking up the pulses from the sender controls the needle position. Most electronic speedos on first switching on send the needle the full arc then normal operation. Perhaps you should check the sender is working before any more regulators are fitted, and of course as previously mentioned any earths are good, fibreglass cars are renowned for it.