The Triumph 2000/2500/2.5 Register Forum

In my experience, most cars speedos seem to over-read when compared with a satnav. The satnav should be pretty accurate when the speed is constant, but I suspect that they will lag behind a bit when the speed is changing. This is probably due to the update rate that the GPS engine uses.

David - that sounds like a good method to check the calibration. I have found that some tachos have a 'pot' inside to adjust the calibration, while some (maybe later ones?) don't. I use a low frequency signal generator coupled to the tacho input and to a digital frequency meter so I can check the signal generator frequency. A bit of calculation gives the frequencies corresponding to revs and so far it seems to be OK!

I've also been trying to find a suitable way to check the calibration of a speedo. The obvious choice is a 'stepper motor' which you can accurately define the motor RPM. Sadly, I've not found a suitable one yet!

Submitting your article to Kevin is by far the best approach. I shall look forward to seeing it!

Cheers,
Mike.

My tacho is the type fitted to many cars in the late-60s/early-70s and it has a trimmer accessed by screwdriver through a hole in the back of the housing (so the instrument has to be outside the panel for adjusting).

Re: Sat-nav accuracy: Last year my son and I went on a 150 mile journey in his car. As 'navigator' I set my Garmin for the trip, however he'd set his TomTom so we let both run for comparison (with his muted). I didn't see any differences in displayed speeds, though I couldn't tell you about lag as I only did spot checks from time to time.

Good article David. I enjoyed that. Of course, being an electronic engineer means I was a bit biased!

Cheers,
Mike.

Mike Stevens wrote:Good article David. I enjoyed that. Of course, being an electronic engineer means I was a bit biased!

Cheers,
Mike.

Thank you, Mike.

Not being at all confident with electronics, I was very pleased - and perhaps a little surprised - that the repair went so well. That tacho should now be fine for another 45 years!

Mike Stevens wrote:In my experience, most cars speedos seem to over-read when compared with a satnav. The satnav should be pretty accurate when the speed is constant, but I suspect that they will lag behind a bit when the speed is changing. This is probably due to the update rate that the GPS engine uses.

Mike.

4 seconds.

Kev