I've had a Telemetrix tacho for a few years now while I was building. It's the electronic 14k rpm one with 6 dip switches on the back. Problem is it's never seemed to read quite right. I always thought it was my set up and figured it would sort itself out when I finally sat down and had a look at it properly.
This afternoon I finally sat down and looked at it. It's now all set up right (nothing was really wrong with the wiring) but it still reads low.

At idle on Power Commander I see 1300rpm, on tacho (4cyl setting) 850ish.
On 6 cyl setting it drops by about a third and at 8 cyl it drops by about half.
At 3300rpm on the PC it reads 2000 on the tacho.

Can I adjust it on my own or will I need to send it away?
Last time I had to send a tacho to Green Gauges for repair it cost me £80 so I'm keen to not have to do that again for something thats never really been right since I got it.

I had a similar problem with Telemetrix Speedo... turns out i was reading the number 1-8 (in your case 1-6) and they were fitted to the board 8-1.... just check them again.

For sure they're the right way round. If I flip them the reading changes as I would expect it to, it's just the actual values that are wrong.

your readings are allmost 2x out, maybe your tacho signal is of a twin spark ignition system and you havent set up the tacho for it? this is what mine was like and i had to fool it by selecting a different number of cylinders.

Just to close this one out....

I spoke to GreenGauges (CaiGauge/SpeedyCables) and after discussing what it was reading they said it was actually the tacho that was at fault. Despite it having been in my garage for 3 years while I was building, Speedy Cables are repairing it under warranty so a good result I think.