So I found my tacho was reading about 3-400 RPM lower than the actual RPM, I couldn't find any resources to fix it other than a whole new instrument cluster.
Here is how I fixed it (pretty dodgy but it worked for me)
Step 1
Determine how far out your tacho is currently reading.
Mine was out about 350RPM at 2000RPM (take multiple readings at different RPM's for the best result...my analyser would break down above 3000, so I couldn't test high RPM accuracy for the tacho)
(NB. if your tacho is out by different amounts at different RPM, this method will not work, either pick the RPM you want displayed accurately, or buy a new Instrument cluster)
Step 2
Remove instrument cluster from car
You have to let the steering column down to slide the instrument cluster past the hazard light switch.
2.a
Remove plastic cover from under dash
2.b
Undo the two torx/large flathead screws that hold the steering column up.
(These are lock-tighted in from factory, I used the biggest flathead screwdriver I have with a wrench attached to the handle)
2.c
Remove the four screws that retain the instrument cluster to the dash. Two on top and two underneath.
(you will need a stubby #2 phillips to get the ones on top, or fold the windscreen forward)
2.d
Slide the instrument cluster forward and undo the plug on each side, and the speedo cable.
(You may have to hold the steering wheel down with one hand. The wiring loom plugs have a 'button' on the outside of each that must be pressed to remove them)
You should now have the Instrument cluster free of the car.
Step 3.
Remove the plastic cover from the face of the Instrument cover
(If your trip meter reset button still has the 'knob' on it, this will likely have to be gently prised off, before you continue)
Remove the 8 screws that hold the 'front' of the cluster on
Step 4.
'Calibrate' the tacho by bending the needle
Now to 'adjust' the tacho... note how far the needle has to travel between the displayed RPM and the actual RPM
(mine was 350RPM out, so about 6mm movement at the end of the needle)
you now very carefully move the needle by hand to the end of its range, until you feel it hit the 'stop' a few mm past 8000RPM
Now very, very carefully continue to twist the needle past this point, the number of mm it is 'out' by, that you previously noted, you will feel the very thin shaft that turns the needle bend.
(if you want to be accurate here (I guessed) put some masking tape under the 0RPM mark (where the needle rests) and mark the resting position, and mark above that a spot that is the distance (degrees) that the tacho is 'out' by.
Bend the needle with the above method, untill the needle 'rests' on the new mark)
(NB... don't bend the needle like in the pic. lay the dash flat and use your thumb and forefinger on the round bit in the middle, and your index finger to apply torque to the needle)
Step 5.
Plug the wiring harness back into the instrument cluster, and start the car to see if your adjustment was successful.
If not, back to step 4.
If yes, unplug the harness again, and put the face of the cluster back on.
(Give it a clean while it's off)
Step 6.
Put it all back together again!
