is there any way of testing how accurate they are?

Can you put a known voltage knowing it should read a certain rpm and check it reads it or is it a bit more complicated?

 

little bit more complex than that, remember they dont read a voltage

 

so if I got one reading 3k when my rs standard rev counter says 2.5k which is more likely to be right?

 

ahh tricky one to answer,,, the standard rev counter at them sort of revs would be accurate, but if you have piggy backed another rev counter off the standard wires then it might be upsetting the standard one!

you can measure the revs with a tidy timing light/strobe techo, hand held tacho