When you say your gauge isn't reading accurately, which gauge isn't right and are you using the correct sender for the gauge?
I do believe for Smith's classic you need the voltage stabilizer, this will supply 3 gauges, how many are connected.
You earthed the stabilizer on the back of the rev counter, is the case of the rev counter earthed? Can you add an earth wire to it.
Are you feeding the rev counter the stabilizer 10 volts? It looks a lot like a Telemetrix rev counter from the rear (which I have) so maybe a 12 volt rev counter and should not need 10v. Check the spec!
Try the stabilizer connected to just the 52mm gauges and make sure it is earthed correctly.
Good luck.