Topic

The Mark one MGF speedometer is notoriously optimistic. The error percentage gets bigger as speed increases. The pointer is returned to zero by a hairspring, like that on a watch and this is retained on the pointer arbor by a split collet. Rotating the collet would increase the resistance of the drag cup to the magnet and that resistance would increase with speed.. From this, I reason that it should be possible to recalibrate the instrument fairly easily. I need to log the Speedo against the GPS, using the tacho as a cross check, then spin the drive at a known speed in the lathe to check the new setting. ( Goes off to find a spare mk 1 Speedo... )

I had reached sixty nine before I was “awarded “ my first speeding ticket granted early one glorious spring morning by a man in a van.
I wasn’t aware of going excessively fast, interested only on road safety along roads devoid of other traffic.
I installed a redundant GPS to cross reference & found my speedo accurate enough to avoid further tickets, providing of course I keep an eye on its reading or avoid the gimlet eye of the local constabulary.

The later electronic speedo is just as optimistic.
More so with 16" wheels than 15", due to slightly smaller overall diameter.
Interestingly, a 16" wheel with a 205/45 tyre gives a much more realistic speedo reading, not that I'm suggesting using non standard tyre sizes.

Personally I always rely on the gps speed on the satnav.

The later electronic speedo is just as optimistic.
More so with 16" wheels than 15", due to slightly smaller overall diameter.
Interestingly, a 16" wheel with a 205/45 tyre gives a much more realistic speedo reading, not that I'm suggesting using non standard tyre sizes.

Personally I always rely on the gps speed on the satnav.

I don't use the Satnav unless I have to because I usually have the roof down. I an perfectly happy to use it for calibration, though. I expect the electronic speedo is a moving coil meter and may be tweakable, too. I plan to set mine up to be 3 MPH fast at 70 which will be fine everywhere else, I expect.

I too will dig the spare speedo from its slumbers & have a play. Firstly I’ll have to calibrate the lath, which runs on three phase, controlled by a inverter drive, to that end I’ll have to find some reflective opto switches (or reed switches), then I’ll have to rag an old calculator & link the 1 & + to a circuit with two optos & an off circuit to another. Then arrange them around the chuck having made an adapter for the speedo drive.
OH shit you do it & I’ll just watch ! !

Nah, know the error from the tach or satnav, put a bit of sqare in the chuck,spin it up, work out the error at the indicated speed and turn the collet a bit until the speed works out right.

Thirty MPH in the workshop & that’s it. I’ve got more jobs on than I can work on at the moment & I’ve kitchen cupboards to build before Christmas.

Carpentry. Before Christmas. Best get on with that then, if you don't want your Christmas dinner burnt...

BAH-HUMBUG!”
You used 2 “C” words…. One of which refers to wood butchery and the other to an obscene festival!

Back to the matter at hand…….

I was wondering how fast you could go in the lathe, obviously the faster you could get it to go the more accurate your fine adjustments will be. To optimal speed being a little above whatever the top legal speed is in your jurisdiction.
Maybe a simpler way might be to use an old speedo cable in a fast drill or a speed adjustable die grinder in conjunction with an optical tachometer to accurately calculate the rpm.
Another advantage of using the cable to drive the speedometer is that you are replicating the real world conditions.
The problem I can see by using the direct drive in the lathe method is that you are leaving out the torsional twist and frictional drag factors of the cable.
In regards to real world conditions, you would be better to rig it up in the car as that would take into account the frictional drag from all the various bends in the cable as installed.
The reason the speedometer needle jumps about at low speed is the cable and it’s various bends, at higher speeds it still suffers from fluctuations but the higher frequency sorta irons them out and the needle can’t react fast enough so as you don’t notice.
That then throws up the problem of getting to the speedometer in order to adjust it….. ah bugger it, I’ll just leave the bastard as it is!

No, Cobber. The speed of the drive at the Speedo head is what matters and it will be turning at the same speed as the other end of the cable.

Wood. God's own plastic. Watchmaker's bench by yours truly in oak, beech and iroko. Just for you Cobber!