The Jaycar unit can adjust any pulsed signal so may work with the rev counter as a last resort.

But if it's a normal volt based swing needle they are easy adjusted anyway ( at least downwards )
You can stick an adjustable pot on the needle actuator, but many rev counters will have a trimming pot on the circuit board anyway, so often there is some adjustment.
If it's a stepper motor then you're screwed, you need the signal adjusted.

As for the speedo, the Jaycar will adjust +/- 100% on the pulses and even an idiot can build it ( I managed it in around an hour, so it must be easy lol )

Setting it up in my case was easy too, the default settings for pulse in/out worked ok, and I just needed to trim my speedo after that.