Tacho wierdness

[shaggy]

One of the few things that has bugged me with my Spider has been that the tacho is highly inaccurate. I have heard many times that they are not the most accurate tachos around, but an indicated 4200 rpm at 100kph in 5th just seems way out. Plus it's kind of noisy (ticking not grinding) and bouncy.

So while giving the car a service and a good look over this morning, we thought we'd check it by using a multimeter (two actually), and this is what we found:

Indicated on Tacho - Actual
Idle:  1100-1200 rpm 750-850 rpm
2000: 1500
3000: 2200
4000: 3100
5000: 3800

The cable looks like it's seen better days, so I'll replace that, but there doesn't seem to be a pattern in way is shows rpm.

So, the question is: is this kind of variance normal?

[Colin Byrne]

Yea i've never seen an accurate mechanical tacho in any of my 105's and i think they have always' shown over as you have measured
I don't have any solution for the accuracy and have ended up putting an electric tacho in the rally car, as for the ticking and bouncy action, pretty sure that is the square drive slipping in the back of the gauge, so a new cable should sort that out.
I've got a feeling that when the friction in the cable gets two high due to lack of lube the inner can "spool up"and then release, causing the needle bouncy and possible showing higher RPM that actual?, just a theory though, and again a new cable should sort that out

[shaggy]

A new cable is on the way, so I'll see if that improves things. I don't mind it being out so much, if it was consistently (or progressively) out I'd be happy for general street driving.

My main problem are those more enthusiastic drives, I want to know where redline is!