A few words on calibration. With the Smiths NEMAG there is no mechanical connection between the road wheels and the speedo needle, it is all done with the spinning magnet drawing the aluminium cup around, so calibration of the speedometer part of the speedo is tenuous. When fitted from new, cars to which had Nemag type of speedometers fitted, the speedo only gave an ‘indication of speed’ not an accurate reading it was not required. It was not until the authorities enforced speed limits with sophisticated equipment that the speedos fitted were found to be generally 5% plus or minus generally being set to read on the fast side. Smiths factory figures were an optimistic -1% +2%. And during calibration the speedo was set to read over speed.
We need to remember that the action of the speedo pointer is governed by the magnetic power of the magnets, a weak set of magnets will give a speedo a slower reading. We should also remember that the magnets are well over 50 years old and might well be expected to be a bit weak.
The smiths calibration machine allowed the factory technician the option of re magnetising the magnets using the electromagnets labled B on the picture either to increase the power of the magnet or decrease the magnets for the reverse. It is possible to recalibrate the speedo by using a home made set of electro magnets and I believe some people have had good results by using the very much more powerful modern set of rare earth magnets to re magnetise the speedo magnets I have not used this method. This operation is a bit time consuming and needs to be done with care. To be honest if you want to get a reasonable speedometer reading and you have a GPS then use that as the guide do not re-case the speedo yet but with some care reconnect the drive system mount speedo in the car and drive at say 30 mph using the GPS as a guide stopping to re set the pointer in relation to the aluminium drag cup this will give and adequate but not perfect result.
The speedo that left the factory all those years ago was designed to work with the correct set of tyres and the correct gearbox ratio, changing the tyres from say 600 x 16 to a larger size will of course give a different reading.
The only part of the speedo head that does have a mechanical link from the wheels is the odometer. On the face of the speedo dial is the TPM number for example 1500. This is the number of revolutions that the drive system needs to advance the odometer by 1 mile. So by setting a drive system up that will give you a set speed of 1500 rpm and timing how long it takes for the odometer to advance by one mile it should take one minute, and that of course is also 60 mph so you can set the speedo needle by trial and error to this speed.
Some suggest using a electric drill running in reverse to give you a speed drive system, I do not recommend this, as in my opinion the drill starts too quickly and could well cause damage to the speedo head. The machine I have designed gives a softer start mimicking the actual action of the car as it gradually gets to 60 mph.
To calibrate the speedo odometer you need to get odometer advancing 1 mile in 1 minute then that it the position that the needle should indicate 60mph simple. But it does take a bit of time to get it right. You can then make sure it is correct by running the speedo drive at 750 rpm and measuring the advanced mile in two minutes the speedo needle should be steady on the 30 mph.