Revvy 3h Gearbox...

[Oldiebut goodie]

I use one to check the revs on heaters - they are as accurate as the ecu is on the newest models. That is operated by magnetic pulses from magnets on the fan.

[Mr Bean]

What does that mean?

I could have misunderstood but I thought it was being suggested that the rev counter reading could in some way be associated with the alternator RPM.
CS

[Oldiebut goodie]

It is totally dependant upon the alternator rpm! Where did you think it was read from?

[marlinowner]

Petrol - rpm from coil
Diesel - rpm from alternator

[shepster]

I could have misunderstood but I thought it was being suggested that the rev counter reading could in some way be associated with the alternator RPM.
CS

That's because it is on a Diesel.

[Mr Bean]

Sorry I just assumed the alternator would be driven by a non-synchronous drive like a "V" or flat belt which would be less accurate/reliable
I just hadn't seen a case where the alternator is driven by a synchronous drive.
Sorry about that. Possible
CS

[Rusty-Bretta]

Alrighty... having returned from our wanders around Northern Spain, I can confirm that the Revs are now reading spot on.
The simple tweek of taking off a few of the rpm and matching it to the drive pulley have corrected the readings. I now sit at 100kph at roughly 3k revs.
Many thanks for all your inputs, time to find something else to fiddle with....