The web page says that the tachometer "runs from a sensor". If you don't want to use the specified sensor, you or someone else will need to reverse engineer the internal circuitry of the tachometer, to figure out how to control the indicator needle.Or, replace the needle motor with one that is Arduino compatible and comes with instructions, like this one.
there are plenty of people on YouTube who seem to have done it
With that exact same gauge?
If it is just emulation you need then a MOSFET running 12 volts through it but triggered from the Arduino to get the pulses would be a good option.See this project for a heads up.Bob.
People around here are always willing to help out, so post what you have exactly and a good description of the exact issues and they will try thier best to get you going.Just remember the devil is in the details.Some may need an ignition type pulse and some may just need an ECU pulse of some description.Working out which is which would be a key factor in success.
If you want to experiment, you might wire the tach up to 12v and check the input wire for voltage to ground with a multimeter. If there is no voltage on the input wire, then you might connect the arduino ground to the tach ground and an arduino digital pin to the tach input wire. Then try doing an analogWrite(digitalpin, 127) to the digital pin and see if you get a reading on the tach. Per the analogWrite reference page, various boards have pulse frequencies that range from 490hz to 1000hz.