mate has recently wired up a aftermarket tacho into his car
wiring appears to be hooked up all correctly not a whole lot to stuff up but the trouble we are having is with the tacho set on 4 cyl the revs are way to high, on 6 cyl slightly lower, and 8 cyl its pretty much bang on at idle and low revs, but still reading to high while driving, maybe a fault with the way its setup but just looking at ideas for where to start with diagnosis
cheers

What brand tacho is it
but you also need to make sure you have a good earth
if its a cheapie
simply put do not waste your time
thay are cheap for a reason

What kind of ignition system / coil?

it was only a 100 dollar drift tacho to my knowledge we thought this was possibly the reason but was wondering if there could be any faults in the way it was hooked up but this is also hard to tell without you guys looking at it, thanks for the advice we shall keep pushing through and hope we work it out